Support for OpenCL 2.0 constants/GetInfo

doc/make_constants.py

pyopencl/cffi_cl.py

setup.py

src/c_wrapper/device.cpp

src/c_wrapper/device.h

src/c_wrapper/kernel.cpp

src/c_wrapper/memory_object.cpp

src/c_wrapper/program.cpp

src/c_wrapper/sampler.cpp

src/c_wrapper/wrap_constants.cpp