Bump minimum required cmake version (fixes #566)
Declare OpenCL kernel arguments as 'const' if they are not written to.
Added HL_OCL_EVENT_DUMP to enable dumping kernel start/end times.

Also checks for overlapping kernel execution.
Show device info if dumping events.
Enable -cl-kernel-arg-info if not NVIDIA.
Merge github.com:Halide/Halide
Merge branch 'ocl-tp-dev' of github.com:jrprice/Halide