Used pyflakes to clean up a bunch of unused imports.
Reformatted whitespace to be PEP8-compatible (used `autopep8 -i -r -j 4 --max-line-length=99 .`)
Minor simplification of Python nineml module structure - turning some packages into modules, removing the unused `util` module.
Storing png images in the repository, so that we can build the docs on systems that don't have the "convert" utility installed.
Missed from last-but-one commit.
Added copyright/license information to the docstring of each Python module.
Importing etree directly rather than indirectly