Minor fixes, git-wip-mode

- Just a very quick fix I made to my local system that I thought I'd
  share.

  I quoted some filenames which fixes paths with spaces (might be more
  elsewhere, this was just enough to get git-wip to work for my
  specific case).

- A new minor mode for Emacs, git-wip-mode.

  Very simple right now. This is so I can disable git-wip on a
  per-buffer basis -- executing git-wip on every save is quite
  annoying as I save often, so I'd like to be able to decide for
  myself if I want to use it or not.

  Also, git-wip-mode runs git-wip asynchronously, which makes for a
  more responsive user experience.