Ability to ignore missing validators, validate multiple at once

We're doing some front-end validation using the same attributes (with Anchor) that we do with waterline on the backend. I made some changes so this can be possible:

 - validating multiple rules at once with Anchor.toss
 - ability to ignore missing validations, so that we can just ignore things that waterline adds, like `unique`.

The only potentially kind of breaking change this uses is that missing validator errors are returned instead of thrown.  They are now handled in the same way that missing types are handled.

Unit tests pass.