A less hack solution to the 180 wrap issue.

Thanks @taldcroft.
Change default representation of solar frames to wrap180.
more small tweaks.
This makes the workaround behave prior to merging astropy/astropy#2691.
Merge pull request #3 from Cadair/coordinates-framework

Merging this one in. Seems fine by me! Brings in much needed 180wrap functionality.
This makes the representation subclassing work using astropy/astropy#2691