Enhance git support

Added a couple of git related modes to the default distribution:

* git-commit-mode
* gitconfig-mode
* gitignore-mode
Add a sensible .projectile file to the project
Shameless self-plug :-)
Fix #273 - added some update instructions
Clean up the modeline a bit with diminish.el
A bit of cleanup and doc-string fixes
Clean up prelude-mode.
Merge branch 'master' of https://github.com/bbatsov/prelude into zeank