[WIP] Adding licensing documentation to the repository.

This change attempts to clarify the licensing position held by the
PHP-FIG group. To do so it-

1. Assigns the MIT license to code,
2. Assigns the CC-BY 3.0 license to documentation,
3. Provides text of those licenses,
4. Updates the bylaws to express these policies,
5. Updates the "Contributing" document with these policies.