Xcode 5 and 64 bit support

[karlvr]

I've compiled GPUImage with Xcode 5 and 64 bit support. Presumably this works :-) This is a significant change in here, and that is that the Xcode project minimum deployment target has changed to iOS 6. This may not suit everyone, but I'm guessing that the majority will care about targeting the new devices.

[BradLarson]

I might hold off on this until I get an actual iPhone 5S in hand. I've done 32->64 bit migration on my Mac applications before, and I know that all kinds of subtle issues can crop up once you run on 64-bit hardware. The GPU in the 5S is also a very different animal, so it will require some closer examination.

[BradLarson]

I'm going to merge this in, but I'll be rolling back the default 64-bit support. Too many people depend on the framework working on targets back to 4.0 to leave that as the default. They can enable this support if needed.

[BradLarson]

I found one last thing throwing precision errors, and tested everything out under 64-bit. Seems solid, because I think I fixed the few actual problems in there when I migrated this to the Mac earlier (with a 64-bit target there).

I did roll back the architectures in the project and my test application, and have the framework once again deploying to 4.0. People would really start to complain if I removed support for older platforms now, but I may drop 4.0 soon.