The issue was that AsyncCandidateRequestReady in the ultisnips completer would
always return false if there were no snippets for the current filetype, leading
to an infinite loop in CompletionsForQuery.
Fixes#270.
This was done by introducing a new ThreadedCompleter class that descends from
Completer. Both JediCompleter and FilenameCompleter descend from
ThreadedCompleter.
This implements the filename completer and introduces integration with
UltiSnips. The user will now see snippets in the completion menu. After
selecting a snippet, the user should invoke the UltiSnips trigger key
(which should be changed from the default of TAB) to trigger the snippet
expansion.
Fixes#77, Fixes#36
