Update contribution info and switch to markdown (as reST is awful).
Add docstrings and a bit reformatting to comply with PEP8 and PEP257.
Add flake8 config.

Add the config to setup.cfg for now but it should be moved to tox.ini as
soon as we have one as it fits there semantically better.
Add ordereddict requirement for Python 2.6 to setup.py. Update Readme.
Fix missing return statement.
Make _add_attribute method protected.
Change exception class to better reflect the error type.
Delete generated MANIFEST and add it to .gitignore.