future: Python 3 support

This pull request introduces Python 3 support via the [future](http://python-future.org) package. It supersedes the older try to introduce a Python 3 support with a single codebase https://github.com/obspy/obspy/pull/203.

This pull request is work in progress, the actual status can be be seen on http://tests.obspy.org/?version=3.3.3.

The motivation for Python 3 is best summarized in this article:
http://python-notes.curiousefficiency.org/en/latest/python3/questions_and_answers.html