Require_authorization is now implemented so as the contract executes and
asserts that a particular account approved the transaction, the chain
asserts that this is so and throws if not.

Also, update the tests, since the auth checks now bring to bear the rule
that an account cannot be used in the same block that creates it. The
tests now comply with this rule.

TODO:
 - Check that all declared authorizations get required by the contract
 - Implement the mapping from user permissions to message types
 - Use mapping of permission to message type in
lookup_minimum_permission