Tentative fix for issue #22: remove the width argument from the functions extract_poly_slice() and extract_pv_slice().

I have updated the example notebook as well, but this needs to be tested.
Merge pull request #27 from bluescarni/bugfix-for-issue-22

Tentative fix for issue #22: remove the width argument from the function...
First draft of docs
Import WCSPath into top-level namespace for API docs
Merge Path and WCSPath
Merge extract_pv_slice and extract_pv_slice_hdu, and refactor
Fix failing tests
Use SpectralCube.read
Renamed pvwcs
Ensure compatibility with Astropy <= 0.3.x
Remove unused files
Removed unused routines.

Pixel scales can now be retrieved with get_spatial_scale and the logic to deal with cases where spacing is specified in pixel or world coordinates is in extract_pv_slice.
Removed old TODOs
Fix case for DS9
Fix Python 3 tests
Fix Python 2 test