Sky coordinates documentation

This is the long-overdue documentation update for `SkyCoord`.

- It includes the 3 commits from #2524 that update the `SkyCoord` docstring and so that will be closed.
- There is one substantive code change (and new test) related to allowing numpy arrays as input coords.
- It includes some editorial changes not strictly relate to `SkyCoord`.
- Locally it is passing all tests and doctests for coordinates on py26, py27, and py33.  We'll see what Travis has to say on linux.