Python 3 only port

This pull request is instead of pull request #3. It should give Python 3 compatibility (minimum 3.1 according to a comment in the C code), but no longer works with earlier (Python 2) versions.

I have cherry-picked and adapted some of @lygstate’s commits from the other pull request, dropping the backwards compatibility bits. I also added my own changes, in particular converting more code to use bytes rather than text strings, and porting the example code. I also added a test for apply_txdelta_window() which would have failed in the previous Python 3 port, and edited the README file.

I have mainly tested this with Python 3.3. All the tests pass (though a few are skipped), and the examples seem to run okay. It also seems to be working well with my own program which mainly uses the _subvertpy.ra_ module. Let me know what you think, if this is the right kind of direction you wanted to go in, etc!