pyplot.savefig doesn't expand ~ (tilde) in path

`pyplot.savefig` throws an error when given a path with a `~` for the user's home directory. Writing out the full path works as expected.

``` python
import matplotlib.pyplot as plt
output = '~/plot.png'
plt.plot(range(10))
plt.savefig(output)

FileNotFoundError: [Errno 2] No such file or directory: '~/plot.png'
```

Matplotlib 1.5.0, Python 3.4.3 (Anaconda 2.4.1), Linux
