dpxdt pip package for local pdiff testing

This creates a pip-installable command line tool which can be used for local testing.

General usage is:

    $ pip install dpxdt
    $ dpxdt update pdifftests/
    ... takes screenshots ...
    $ dpxdt test pdifftests/
    ... runs pdiffs ...

It's pretty basic, but I've already found it stupendously useful for [comparea](http://www.comparea.org), for example.