LaTeX completer for YouCompleteMe

This is the initial import. The LaTeX completer improves over YCM's
standard identifier-based completer by adding completions for \ref{} and
\cite{} statements.

* Citations are taken from the *.bib files located in the current
  working directory and extracting the BIB elements' labels.
* Ref targets are found by scanning the *.tex files in the current
  working directory for \label{} statements.
Merge branch 'master' of github.com:bjoernd/vim-ycm-tex