pickup __version__ without importing from xmlrunner.

importing xmlrunner causes a race condition while doing the setup.
if later we add requirements that are missing, then importing xmlrunner will
end up in an ImportError. setup.py needs to not depend on the same dependencies
as xmlrunner (iow. just some generic code specific to distutils/setuptools.)

Also use semantic versioning; __version__ is what PEP8 recommends.
distutils.version has classes to parse version strings if someone ever wants
to compare version strings.

see http://stackoverflow.com/questions/2058802/