Parallel test

These changes add the '--parallel N' option to run.py for the test suite ( #788 ).  In order for it to work, you'll need to install some additional python modules as described in the 'Parallel testing' section of:

  https://github.com/wiredtiger/wiredtiger/wiki/HOWTO:RunTheTestSuite

Before installing these, please verify that run.py continues to work without the --parallel option.  That is, basic non-parallel testing should still be possible without any additional modules.  That sort of testing is also possible with python's -S option, which has python forget about anything site-installed.