Inline `_instance_from`
Replace specific validators with partial function application
Inline `_key_from`
Refactor model API

I've basically just renamed functions to something that makes more sense to me. Instead of talking about keys, url_keys, url_safe_keys, etc., we just talk about ids, which are defined to be url-safe.
Define `random_socket` in terms of `random_socket_from`
Simplify validators by using implicit None return value
Testing /edit/.
Merge branch 'code-review' of git://github.com/zachallaun/brevity into zachallaun-code-review
Merge branch 'zachallaun-code-review'

Conflicts:
	brevity/application.py