Add build/commit date (since __version__ is not maintained)

It's very hard to track down which PySide2 version you're running since PySide2._version_ isn't being maintained and doesn't change.

I'd like to propose that until the _version_ is being maintained an automatic build date is put in e.g. PySide2._build_date_ or PySide2._commit_date_ and could be generated automatically off the following code:

>>> from datetime import datetime
>>> str(datetime.now())
'2017-08-08 14:49:57.979339'

By having that, users can more easily accommodate for changes in PySide2 versions more transparently, making their code much more readable.