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.
Some long overdue keybinding doc improvements
Minor doc fixes
Mention prelude-ensure-module-deps in the README
Merge remote-tracking branch 'bbatsov-prelude/master' into 'aisbaa-prelude/master'