Improve documentation.

More code doc through docstrings and doctest.
Minor updates to the MANUAL.
Detect pytest-xdist instead of crashing :)
Comment and pep8 validation for TestEnv class
Improve debug of failing unit tests (TestEnv)

- Catch TypeError exception from assertAlmostEqual() since
  we can't do anything there and raise a "mismatch" message
  instead
- Display Python and Pythran results on separate lines
Fix EnvironmentError exception handling

The String conversion were incomplete, has to
generate a "tuple"-like output and not a list
when argcount > 3.
I'm unsure if the right fix wouldn't be that
BaseException should rather store the arguments
as a tuple instead of a list.
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