Added OpenCL error names based on #define-s from cl.h
Merge pull request #2 from vmarkovtsev/master

Added OpenCL error names based on #define-s from cl.h
Changed some cffi API calls, renamed Program.src -> Program.source
Added Program properties from clGetProgramInfo
Merge pull request #3 from vmarkovtsev/master

Added Program properties from clGetProgramInfo
Set PYOPENCL_CTX to 0:0 only if it is not already set