Refactor top-level files

[molinav]

This pull request adds some minor changes to the package top-level files:

• The content in .requirements-2.6.txt (i.e. unittest2) is moved to the standard requirements-dev.txt.
• The MANIFEST.in is updated according to the current status of the library.
• Changelog is renamed to CHANGELOG.
• The license text appearing in the README.md file is also copied into a LICENSE file for clarification.
• The Markdown syntax in the README.md file is slightly modified.