[Rivet] Python 3

[David Grellscheid]

Hi all,

I've gone through the core features of YODA and Rivet, and made them
Python-3-compatible on the default branches. I have not tested all
helper scripts yet, so if you notice anything odd, please let me know,
and I'll fix it. I'll go through everything in the next days.

Also, I haven't run a python 2.6 test yet, most likely the UI is broken
for such old pythons now, and I'm not sure how much effort the
compatibility would be yet, we'll need to see if it's even worth trying.
Python 2.7.3 (April 2012) onwards should work.

See you,

  David