Add first version of API docs

[cdeil]

At PyAstro16 @joleroi set up online docs for this regions repo here:
http://astropy-regions.readthedocs.io/en/latest/

This pull request adds a first version of the API docs.

There's a few warnings in the API docs build like this that I don't know how to fix.
@keflavich @astrofrog or @bsipocz - Do you know how to address those? If yes, can you make a PR against this PR?

<autosummary>:None: WARNING: toctree contains reference to nonexisting document 'api/regions.shapes.PixelRegion'
<autosummary>:None: WARNING: toctree contains reference to nonexisting document 'api/regions.shapes.SkyRegion'
None:None: WARNING: toctree contains reference to nonexisting document 'api/regions.shapes.PixelRegion'
None:None: WARNING: toctree contains reference to nonexisting document 'api/regions.shapes.SkyRegion'

https://gist.github.com/cdeil/539490416ffee012f3a1081b90940168

If I don't hear back I might merge this soon, because something with warnings is better than nothing. :-)

[cdeil]

How about pulling in regions.core, regions.io and regions.shapes into the top-level regions namespace? I don't think the sub-package hierarchy is very useful here because the package isn't terribly large?

[astrofrog]

@cdeil - I quite like the hierarchy in terms of the code files, but I have no objections to importing the classes into the top-level in terms of public API.

[cdeil]

@cdeil - I quite like the hierarchy in terms of the code files, but I have no objections to importing the classes into the top-level in terms of public API.

@astrofrog - I'm making this change now.

If anyone objects, please speak up now or later ... we can always change back should people be unhappy with the simple single-namespace solution.

[cdeil]

OK?

[larrybradley]

👍

[cdeil]

I'm merging this now.

If you only see this later: Feedback is still very welcome!