Add jedi-based completion.

*note: this isn't really ready for prime-time, but I wanted to get the discussion started*

This first version only uses the Jedi completion engine after a ".", similar to
how the ClangCompleter works. It is also entirely synchronous and blocks for
quite a while the first time it is called.

It also pulls "jedI" in as a submodule (the same way jedi-vim currently does it).
This works fine with vundle/pathogen, but I don't know if you'd rather copy it in
to the repo or install it some other way.