Added syntax highlightning for tmux.conf
test presence of persistent_undo module before use it. (prevent error msg for versions<703)
just a spelling mistake in comment
Merge pull request #406 from jeremyclement/master

Resolve error "E518: Unknown option: undofile" for vim < 7.3
Merge pull request #405 from jby/add_tmux_vim_syntax

Added syntax highlightning for tmux.conf