- Allows to access some internal Octave functions

Fixed compilation issue with Octave 4.0.2 (undefined OCTINTERP_API)

- Addressing build error: ‘OCTINTERP_API’ does not name a type
- inserted #include <octave/config.h> in swig_octave_version.h

Working towards ocave 4.3.0+ compatibility

- Bug with current octave_main start of embedded interpreter,
  when determining loadpath during interpreter initialization,
  the interpreter of the global octave app is required, but
  not yet initialized raising exception about missing context.

Octave 4.3.0+ compatibility update

"0+" patch version issues:

- changed SWIG_OCTAVE_PREREQ version check macro to ignore
  patch version "0+", to workaround C preprocessor errors
- override R's utils::compareVersion function in R/utils.R

Octave interpreter startup API update:

- creating octave::cli_application before calling octave_main
  to resolve problem of missing global octave application,
  when creating an embedded octave interpreter.
  This is due to an Octave API update.

compatibility for Octave version >= 4.0 < 4.3

- octave/config.h has to be included before octave/version.h
  for a certain range of octave versions

Octave 3.8.2 compatibility

more compatibility for R 3.8.2 and beyond

Octave 4.3.0+ works

- Creation of an embedded interpreter in this version of
  Octave is a bit strange, working around a possible bug
  in the not yet released version 4.3.0
- An octave::cli_application has to be created first, and its
  interpreter then created and initialized, then a separate
  embedded interpreter can be produced and executed without
  waiting for a command prompt. see src/rcpp_octave.cpp
- Creating an embedded interpreter without an application
  results in an exception when determining the load path.

Octave 4.2.1 works, too (with 3 new workarounds)

- octave_main cannot be used anymore to create the interpreter,
  the embedded application is destructed upon exit from that function,
  deleting the interpreter, which sets the global variable
  octave::current_evaluator to NULL and causes SEGFAULT in feval.
  resolved by creating embedded_application* inside rcpp_octave.cpp
- octave.cc (v4.2, 4.3+) cmdline_options::cmdline_options() uses
  argv + octave_optind_wrapper(), where argv is the char** produced
  in rcpp_octave.cpp octave_session and optind is a global int
  giving the number of arguments getopt.h reports for the executable,
  having nothing to do with the args produced here. If the executable
  was called with more than narg arguments, this gives a SEGFAULT.
  resolved: octave_session now ensures that char** is long enough
- upon exit, unloading the symbols, the destructor of
  octave::symbol_table erases invalid octave_user_function handles,
  resulting in SIGBUS. Resolved by triggering cleanup earlier
  in octave_session. Octave 4.3.0+ does not have this problem anymore.

- Octave error messages are passed on with more detail info
- updated Octave error message string verification in 3 unit tests
- changed rcpp(...) call to cxxfunction(...) call in interface.R
- added 'inline' to list of required packages in DESCRIPTION

- removed qlibrary('inline') from unit test, due to missing pkgmaker export

- Docker container tags must be lower-case
- Run build command inside the container only

- .travis.yml updated to perform builds using layered docker containers
- base image created using docker hub account docsteb:
  docker build dockerfiles/trusty-r-texlive -t docsteb/trusty-r-texlive
  docker push docsteb/trusty-r-texlive
- different Octave installations are layers on top of base image, e.g.
  docker build -f dockerfiles/octave4.2/Dockerfile .
- Octave 4.2 requires extra packages for linking:
  The packages added to dockerfiles/octave4.2/Dockerfile
  should probably have been brought in automatically when installing
  liboctave4.2-dev from ppa:kwwette/octaves
- built tar.gz package file is copied out of the builder container
  once completed