Add sources for building Sphinx documentation.
Mock C-extensions out for doc building.
Also mock ipaddr module for doc builds.
Include Sphinx documentation for gen_bridge_descriptors script.
Fix Sphinx references to bridgedb.parse and bridgedb.Storage.
Remove unused static resources from doc/sphinx/source/.
Add modified Bootwatch stylesheet so we don't hit googlefonts.com.
Link twisted.* Sphinx directives to Twisted API docs.
fixup! Add modified Bootwatch stylesheet so we don't hit googlefonts.com.
Add Sphinx options for ignoring pickled doctree state.
Update Sphinx theme and add todo directives.
Remove the TODO header in doc builds for index.rst.

The sphinx.ext.todo seem not to be working…

It is supposed to work, I believe, by putting the following into a
docstring:

    class Foo(object):
        """A foo.

        .. todo:: Do something about bar.
        """

And then the `.. todolist::` directive in Sphinx will find all such
`.. todo::` directives in your code, and put them in a list (the way I
was doing it, these would be on the index.html page) with links to where
the `.. todo::` is in the source code.

However, this seems to *only* work in docstrings proper in Python, the
following do *not* get picked up by the `.. todolist::` Sphinx
directive:

    * Normal Python inline comments, i.e.:
        # .. todo:: There is this thing we should do.

    * Sphinx attribute inline comments, i.e.:
        #: .. todo:: There is also another thing we should do.

Because all of the TODOs in BridgeDB are currently within normal Python
inline comments, the `.. todolist::` on the index page doesn't include
anything, and thus there is an awkward looking, giant, blank header with
nothing underneath it. Thus, I'm taking out the header, but leaving in
the `.. todolist::` directive (so that we *can* use `.. todo::` in a
proper docstring in the future and it will be automatically included in
the documentation here).
Merge branch 'feature/sphinx-docs' into develop