added the module for fast implementation of the GrowCut method.

Hi JC, 

I followed the instruction in [1] to add the fast GrowCut module. Could you help to check if it can be merged into the master now?

Thanks a lot,

Liangjia

[1] http://wiki.slicer.org/slicerWiki/index.php/Documentation/Nightly/Developers/Tutorials/Contribute_Extension_Description_File