[#24] Add fugitive.vim

Features include:

* Gblame for interactive vertical split with git blame output.
* Ggrep to search the work tree (or any arbitrary commit) with git grep,
  skipping over that which is not tracked in the repository.
* :Glog loads all previous revisions of a file into the quickfix list so
  you can iterate over them and watch the file evolve.
* :Gbrowse to open the current file on GitHub, with optional line range
  (try it in visual mode).