Make a draft for a changelog

This is mainly meant for v0.2 but also includes a stub for describing the minor
changes between v0.1 and v0.1.1. v0.1 is simply considered as initial release.
Include CHANGELOG in source distribution
Add the CHANGELOG to the PyPI front page

Make it more easy for PyPI users to assess relevant changes!
Make sectioning in CHANGELOG compatible with README

Allow textual concatenation to make the CHANGELOG a subsection of the
README.
Clarify wording regarding signatures
Merge pull request #37 from coldfix/changelog

Changelog
Issue a warning about missing documentation files

Both README.rst and CHANGELOG.rst are relevant to create the appropriate long
description required for PyPI. In case one of them is missing for whatever
reason, the user should see a warning about this problem.
Merge pull request #38 from aepsil0n/docs/changelog

Changelog for versions up to v0.2