[test] Added unit tests

- Moved the TransactionNotFound exception to a more appropriate place.
- Changed the serialization for AccountingExceptions
- Override the Exception.__init__ method in AccountingException
- Added __eq__ methods to accounting.models.*
- Catch the TransactionNotFound exception in transaction_get and return a
404 instead. This could be improved, perhaps in the jsonify_exceptions
decorator so that all endpoints that raise a TransactionNotFound
exception automatically return a 404.