Many small bugfixes, working on a full testsuite.
Further bugfixes, new `falsy` parameter to `Sentinel` class allows constructed `Sentinel`(s) to evaluate as falsy.
Fixed bugs in `to_dict`, `to_json`, `__iter__` and `_get_value`.
Proper support for recursive key ancestry. Working `urlsafe` and `flatten`. Massive improvements to efficience in all Key-related classes.
55-test suite for the new Model API! :)