README: Document dependencies (IPython, PIL, pytest)
README.rst: Require pytest >= 2.3

The @pytest.fixture decorator was introduced in pytest-2.3 [1].

[1]: http://pytest.org/latest/funcarg_compare.html#direct-parametrization-of-funcarg-resource-factories
README.rst: Add Python dependency (>= 2.6)

Python versions 2.5 and earlier lack support for the '{0}'.format(...)
syntax.