Phase 1 of reorganization

@leejjoon - this is mainly moving files around to confirm to the typical layout for affiliated packages, specifically:

* ``doc`` -> ``docs``
* ``tests`` -> ``pyregion/tests``
* ``src`` -> ``pyregion/src`` (in principle we should name it something more explicit than ``src`` but this will do for now)
* moved all tests that were in modules into ``pyregion/tests``

I think there is a fair bit of code (e.g. related to angle formatting) that can be refactored to use Astropy instead, and I also plan to switch over to the astropy affiliated package template, but thought I'd do one step at a time.