Improve example `demo_elementwise.py`.
Improve example `demo.py`.
Merge pull request #49 from gw0/fix-docs-01

Improved examples in documentation
Fix warning for `cl_khr_fp64` in OpenCL 1.2.
Merge pull request #50 from gw0/fix-warning-fp64

Fix warning for `cl_khr_fp64` in OpenCL 1.2
skip tests unsupported by pocl
test enqueue_barrier/marker and wait_for_events
test unload_platform_compiler
change submodule address