Change plotting examples from pywcsgrid2 to wcsaxes

There's a few examples that use `pywcsgrid2` and the install instructions talk about it:
https://github.com/astropy/pyregion/search?q=pywcsgrid2&type=Code&utf8=%E2%9C%93

I guess this should be changed to [wcsaxes](http://wcsaxes.readthedocs.org/en/latest/) or [aplpy](http://aplpy.readthedocs.org/en/latest/)?
@astrofrog This doesn't work:
```python
ImageGrid(..., axes_class=(WCSAxes, dict(wcs=wcs))))
```
I've never used `ImageGrid` ... do you know how to do this?