Add support for defining actions

[kylef]

As a user of rivr-rest, I should be able to define non-idempotent actions/transitions on a resource

def get_transitions(self):
    return ('create', 'delete',)

def create_attributes(self):
    return (
        StringAttribute('question', required=True),
    )

rivr-rest-peewee will be able to automatically create create/delete/update transitions based on the database.