Add wrapper for the clEnqueueNativeKernel() function, closes #5
Update docs to reflect commit f70849d
i.e. implementation of a wrapper for the clEnqueueNativeKernel()
OpenCL function.
Add test for ccl_kernel_enqueue_native() function

- Test working with Intel and AMD CPU OCL implementations on
Linux.
- Test checks if device has CL_EXEC_NATIVE_KERNEL execution
capability, so it doesn't fail with unsupported devices such as
GPUs.
- Test will not work with OCL stub because clEnqueueNativeKernel()
is not yet implemented in the OCL stub.
Add clEnqueueNativeKernel() to OCL stub

The test for ccl_kernel_enqueue_native() function now works with
the OpenCL stub.
Add basic error checks to clEnqueueNativeKernel() in OCL stub