Open keybindings by platform

Last thing I promise :)

Testing the package in both Sublime Text 2 and 3 it seems to work fine, but the Keybinding are a bit counterintuitive, `Keybindings - User` opens the default options, and `Keybinding - Default` opens a new keybinding file.

I only changed the JSON that defines them so it works like the other packages (Default = Package default, User = User by platform).

Thanks for all the feedback (and patience) and for merging the commands!