Load custom rake scripts from sub-directories to allow using sub-modules

This is my first pull request so please be gentle :wink:

I modified the Rakefile a bit to make it parse for *.rake files in sub-directories of _rake/ which was necessary for using sub-modules.

However I don't know enough about the Dir class and its globbing so this could be a total mistake. Anyhow, I think this feature would be neat because I would try and make all my extensions to JB sub-modules in the future.

Regards, Enno