Feature/python 2+3

(DRAFT – do not merge just yet.)

With the help of `six`, we’re able to run both Python 2.7 and 3.3 from the same repository. Please note that the code does not work identically. Views and iterables behave differently and may even misbehave in either version, potentially depending on collection sizes. In turn, one version might be much faster than the other and both might be much slower than an optimised version.

Most unicode/binary issues are taken care of by the `pyzmq` library, which makes it unclear whether the networking layer is compatible.

## IMPORTANT

There are some strange things going on in commit bcfb0ba. Without the `context.destroy()`, the test suite does not complete with Python 3.3 but blocks some time later during some other, potentially unrelated test. (CTRL+C hints at `context.__del__`.) Renaming `context` to `self.context` and thereby keeping it from being garbage collected does also help. My guess is that some code in `context.__del__` does the blocking while trying to shutdown other contexts in other threads. We need to figure out whether this is true and whether we need to `destroy` all other contexts as well.