Improve OpenMP support.

* More robust parsing of directives.
* Add support for OpenMP runtime library.
* Handle conflicts with c++ keyword private THXS to Mehdi Amini's report.
* Refactor OpenMP stuff into a new module.
* Improve testbed based on OpenMP Validation Testsuite.
* Distribute a Python binding of libgomp.
Fix a few OpenMP bugs.

* Allow to attach directives on Pass nodes.
* Make sure OMP directives are not lost/misplaced when held by the final
  statement of a block.
Fix sections for OpenMP support
Add sleep function from time module
Add OpenMP tests