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.
Sounds really usfull for me too1 I haven't yet come into the problem of making releases from different branches (only from the mainline) but I agree this might be a totally valid need! But would you agree that it should be optional?