Support for readthedocs.org

- Moving this into main repo so that commits to phpspec trigger builds.
- PRs against phpspec may now include documentation changes that match the change
- Updated README.md to be README.rst so that the readme can be included in the manual.
- Updated .gitattributes to ignore docs/ folder for purposes of tagging.
- See http://phpspec.readthedocs.org for initial version.