Added acquire_release static test
Moved cl.h to mini_cl.h, with mini cl_platform.h
Added -D flags for static tests when testing OpenCL/PTX targets
Implemented custom contexts for OpenCL/CUDA
Merge branch 'acquire_release_test'
Merge pull request #315 from dsharletg/master

Adds static test for halide_acquire/halide_release using custom GPU contexts