Add NDEBUG to Release builds
Build with SSE on 64-bit
Use num_channels instead of gStuff->planes

One describes the document you're creating, the other the document you
have. If you have alpha channels but don't use them, this will cause
problems.

Found this bug while using SuperPNG to update the website...about
SuperPNG. Eating your own dog food is good.
Updated ReadMe
GitHub doesn't respect my carriage returns
Pull in bug fix and other stuff
Add USE_SSE on Xcode 5

Verified that this is the same as USE_SSE=1
Add NDEBUG to release builds

To keep the asserts quiet.