Simplified Import Procedure

Markdown extensions work best if named mdx_something, so that clients can import with the simple markdown extension name 'something'.

I made various changes to make that work with `markdown-checklist`. This requires a slightly different strategy for meta information like version info, because `setup.py` runs before all the package dependencies are available/resolvable. Also added `.gitignore`, `tox.ini`, `pytest.ini`, and made corresponding changes to `README` and `Makefile`. All modifications tested across py26, py27, py32, py33, and pypy.