Make vcs segments *much* faster

Do not call vcs status unless the relevant files have changed.
Uses inotify, if available, otherwise falls back to stat() based
monitoring. Fixes #396
Fix tests failing if inotify not available
Forgot to move the import to module level
Turn off status colors by default in vim as well
Add a debug() function to allow command line debugging of the vcs code
Merge remote-tracking branch 'kovidgoyal/vcs' into develop