added pep440 compliance option

Hi, we have added some functionalty to replace the version string git commit number with a more pep440 compliant 'dev'. There is also the option to replace 'dev' with a string of your choice (e.g. 'post'). This can be configured via the two new header.py variables:

    pep440_compliant (default False)
    release_type_string (default 'dev')

We can see there is some major refactoring in progress in the 'template' branch, but we hoped it would be possible to incorporate these small updates to the master. No test for 'git2pep440', but can be added if you agree to incorporate the changes.
just to get it right, you mean adding +revname (e.g. g12345) as local version? What would that add? Or just as convenience for finding the git revision by the version name? But that would not be the intension of PEP local versions. I have't looked into it deeper, just by looking at the functions I doubt that there are good unit tests prooving what could happen there, so maybe I'm just wrong what the outcome of your suggestion would be...
But in principle I agree