Compile in 64bit, and remove 64 suffix from library on OSX. Fixes #356

Since CUDA 3.1 there is support for 64bit on OSX, so I think it's time to remove the -m32 from the build script.
I've also set the correct -L path on OSX for a default CUDA install (/usr/local/cuda/lib not /usr/local/cuda/lib64).