Aplpy+wcsaxes

Tasks left:
- [ ] Fix pix2world transformations for datasets with more than 2 dimensions. Because of this, overlaying shapes and displaying world coordinates in interactive mode does not work for data cubes etc.
- [x] Add WCSAxes to travis.yml. This branch only works with the latest branch of WCSAxes. 
- [x] Fix ```save``` method. Currently using ```bbox_extra_artists``` somehow causes only part of the image to be in the saved figure. EDIT - no need to look into this anymore.
- [x] Fix changing spacing for grids. EDIT - was a bug in WCSAxes, should be fixed with https://github.com/astrofrog/wcsaxes/pull/106
- [x] Fix ```set_style``` method. Style 'plain' sets the wrong separators.