disable get_host_array for MemoryObject without USE_HOST_PTR since the returned array may not be valid

Add `out` parameter to ReductionKernel call.

Add some tests for wrapper backend and skip some tests that are not supported by pocl.

Fix warning for `cl_khr_fp64` in OpenCL 1.2

Improved examples in documentation

Fix AttributeError: 'Win32Platform' object has no attribute 'GLX'

Rename pyopencl.ipython to pyopencl.ipython_ext to convince 2to3 that 'from IPython import' is not package-local