Add MacOS X support.

Hello,

I've made some modifications to support MacOS X - in particular it appears that type_name<size_t> does not get defined, so I made a template specialisation for that case; and I included CL/cl.hpp because MacOS X's in-built OpenCL headers do not include it.
Other changes include FindOpenCL.cmake to add support for find_package (public domain from http://gitorious.org/findopencl).

Cheers!