Make use of Matplotlib's projection= framework

This makes the initialization of WCSAxes much easier:

```
import matplotlib.pyplot as plt
from wcsaxes import datasets, WCS

hdu = datasets.fetch_msx_hdu()
wcs = WCS(hdu.header)

fig = plt.figure()
ax = fig.add_subplot(1, 1, 1, projection=wcs)
```

@cdeil - could you review the changes to the docs since you raise this before?

cc @anizami 
Is this a temp solution and you'll change to the Astropy WCS class soon?

If this will be used for the next year by end-users you should add a docstring explaining why there's an extended `WCS` class.