Allow generic restrictions upon credentials

[therealbstern]

This is a superset of #33 (make usernames more restrictive). The premise is that we want to tell the Authenticator that passwords (or usernames, or X.509 certificates, or ...) need to have certain criteria before they can be accepted.

The trivial way to do this is to implement a trait where the default implementation is to accept the value, and it can be overridden with an implementation that knows better.

[therealbstern]

Ideally, the Username should have a trait associated with each store type and presumably a method to convert usernames between backing store types.

[therealbstern]

I would like to fluff and fold so that we could provide overlapping sets of traits, maybe a Vec of rules that must be satisfied, and the library enumerates them, but the lower-hanging fruit is to allow the consumer of the library to implement the trait in whole cloth.