Changing xrange to six.range for PY3 compatibility
Make composite and base units equivalent.
Fixing documentation links, warnings in wcs
Added APE5 API tests
Added initial files for representation classes
Implement BaseRepresentation and 3 inherited Representations.

Implemented CartesianRepresentation, SphericalRepresentation and CylindricalRepresentation.
Implemented transformations between the three Representations by to_cartesian and from_cartesian methods.

Contains contributions by @astrofrog.
Added example of creating a Distance object from a Distance object.
Added initial tests for SphericalRepresentation
Added tests for SphericalRepresentation
Added tests for CartesianRepresentation
Added test for cartesian to/from spherical transformation
Added missing @classmethod
Fixed cartesian to spherical conversion
Fixed failing tests
Fixed copy= tests
Fixed typos
Added future imports and explanation at the top regarding the transformations.
Added tests for cylindrical conversion, and fixed failures.
Added PhysicsSphericalRepresentation
Added __all__