Added future imports and explanation at the top regarding the transformations.
Added tests for cylindrical conversion, and fixed failures.
Added PhysicsSphericalRepresentation
Added __all__
Add CartesianRepresentation.xyz property
Improved docstrings
Added API improvement suggestions
Convert distance to quantity just once
Add initilization of CartesianRepresentation from a 3xN array
Add fail tests to CartesianRepresentation being passed xyz array
Fix tests for copy=False
Fix exception message checking for Python 2
Added a mention of the rotated() method in the comments
Test ALL THE THINGS

Achievement Unlocked: 100% test coverage
Catch ClyndricalRepresentation rho not length
Use Distance.view(Quantity) to convert distance to quantity
Allow representations to use any unit, not just length, for non-angular coordinates
Added UnitSphericalRepresentation
Added round-tripping test for UnitSphericalRepresentation
Use class method only for from_representation