Create build dir if not already there (e.g. fresh download)
Accept folder names as arguments to do2to3.py
Simplify Python 3 statements under TravisCI, remove tail calls.

The tail calls are not really needed with the improved TravisCI
interface which collaspes each command's output by default.
Keeping the full output is useful when things break.
Run 2to3 separately from setup.py to try and appease TravisCI limits.

The idea is rather than one very slow step (setup.py install including 2to3),
we'll have several slow steps (do2to3 for Bio, do2to3 for Tests, etc, and
only then call setup.py install).