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
A lot of these objects have two public names now.  eg `constantly.NamedConstant` and `constantly.constants.NamedConstant`.  Why make `constantly.constants` public?