Make prettyplotlib Python 3 compatible

These patches add Python 3 compatibility to prettyplotlib. They may not be completely sufficient to guarantee full compatibility and there may be some lurking issues, however, the test scripts are passing on Python 3.3.2 on OS X 10.8.5 (as well as Python 2.7.5, still).

The number of lines of code that had to change to make the codebase run under both Python 2.7 and 3.3 was actually fairly small. In the future, for compatibility, authors should try to use forward-compatible language features (e.g., the `next()` function) in place of older idioms.

This pull request includes a branch created to increase the richness of the package description through PyPI trove classifiers. These will be displayed the next time a new version package is uploaded to PyPI, along with the Python 3 compatibility stamp.