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
Add prelude-update command
Add a note about enabling modules in the README
Add a couple of jump-to-def keybindings
Display a message when loading the personal config
Cleaned up some requires here and there
Open .jbuilder files in ruby-mode
C-M-h is more useful as mark-defun
Minor rewording
Remove some problematic menu-bar-mode logic
A few minor changes regarding OSX support