added the facility to overide resource loading to a custom NSBundle

Hi guys,

We had a requirement to be able to load resources from a custom location outside of the main application bundle on a mac, this is good for level editors and the like.

I've added a method that sets the bundle that CCFIleUtils pulls from. By default this is of course [NSBundle mainBundle] but this can now be overridden using:

[CCFileUtils setBundle:*bundle*];

All the best and thanks for doing such a good job on the project.

Nick