fix unique=True in fields

Makes “ensure indexes” independent of when the MetaClass__new__ is
executed. (indexes are ensured upon getDatabase(), or upon MetaClass
__new__ if the database has already been connected.)
This allows only a single call to ensure_index (pymongo), rather than
one per Document Class creation.
It also adds in indexes for the unique=True property, which has never
been honoured by MongORM.