add SkyCoord.position_angle and associated test

This closes #2311 by adding a `position_angle` method to `SkyCoord`.

This is pretty straightforward, and #2311 was delayed to await #2422 , but this is still a new feature.  So it depends on how strict we want to consider "feature freeze" for deciding if this is 0.4 vs 1.0 .  Of course I'm biased as the author, so I'll leave it to @astrofrog to decide.

cc @richardgmcmahon @cdeil