Allow Py2.5 tests to use pip insecurely
Add install_requires back into setup.py

This isn't a valid distutils setup option, but pip doesn't pick up the
requirements via the old requires parameter. I should probably just
be done with it and switch to setuptools.

Refs #41
Update readme to recommend pillow over PIL
Import setuptools for install_requires.

install_requires is a setuptools option, and using it with distutils will
only trigger a warning or a fatal error (in Python 3.3).
Make pymaging test optional.

Skip the pymaging test if pymaging-png is not installed.