Split Constantly out into its own project.

This does the following:

- Updates code references
- Adds Tox, for py26, py27, pypy, coverage and pyflakes
- Adds a basic README and LICENSE
- Adds a setup.py
- Adds versioneer
This isn't actually the right name.  Private implementation details like `_Constant` didn't get imported and re-exposed in `constantly`.  This should be `constantly.constants._Constant.__lt__`.