Updated TODO
A bunch of updates:

* Added a Zones type
* Added Contains() to the Zones and Users type
* Moved the database test stub code into the dbtest package, renamed database_test to dbtest
* Simplified the database code for deleting objects so we don't have to have a separate function for each type
* Made the model.Delete* functions take in an object rather than an ID, added convenience functions that take in the ID
* Started work on unit tests for the model code
* Renamed model.characters to model.chars due to excessive OCD
* Some improved code reuse in the model
* Removed some unused code: model.MoveRoomsToZone()
* Removed some code that used to refer to the zone with a blank ID which we don't have anymore