Updated fbone/user/models.py for SQLAlchemy 0.8+

Mutabletype has been replaced by mutable in SQLAlchemy 0.8+
http://docs.sqlalchemy.org/en/latest/changelog/migration_08.html?highlight=mutabletype#mutabletype
This add compatibility with SQLAlchemy 0.8+
Merge pull request #6 from wassname/patch-1

Updated fbone/user/models.py for SQLAlchemy 0.8+