Make sure variants work with sphinx

[pganssle]

The way variants work now it will probably work with sphinx. We may want to add tests in at least one travis branch for this.

[pganssle]

I created a basic example module and unfortunately it doesn't seem to work automatically. I'm going to postpone this to a later feature release.

I believe the way forward is:

1. Figure out how sphinx automodule works and see if we can emulate what they are looking for well enough to have to do anything else.
2. If we can't do that (or possibly even if we can), write a sphinx extension that is "variant aware" and does this for us.

For the second item, we can either keep it in the module itself so that it is automatically included (not particularly clean), or as a separate module (my preference).

[pganssle]

OK, this is resolved with the creation of the sphinx_autodoc_variants extension.