update findContainerFor() to work on core mods

Hello,

Here's a pull request so that core mods can register entities,guis,etc. Right now, the registrations methods crash because findContainerFor() returns null on core mods.

I started a thread on the forums about it. You can find more info there.
http://www.minecraftforge.net/forum/index.php/topic,11343.0.html

Thanks,
Cuchaz