Add jedi-based completion.

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.
Refactor jedi-completer to use a worker thread
Note about <C-U> being effectively broken.

Related to issue #24.
Bug fixes for the new python semantic completer
Python code style fixes
Posting an error message when jedi import fails
More style fixes
Making the python thread not hang Vim on exit

As the Pytho docs say, "The entire Python program exits when no alive non-daemon
threads are left." Basically, the daemon threads are safely killed.
Merge branch 'grncdr-jedi' into dev
More style fixes
Updated docs noting the new Jedi engine for Python
More cleanup for correctness and simplicity

The jedi completer now won't hang on <C-Space>