Making sure that <C-Space> works in console Vim

Fixes #256.
FilterAndSortCandidates returns all on empty query

Previously it returned an empty list. It makes mores sense to return the input
list of candidates because conceptually everything matches an empty query.
Add general completers support
Add general UltiSnips snippet completer
Add general filename completer
remove useless comment
names refactoring
fix listdir() path problem
update to upstream FilterAndSortCandidates version
remove unused import