Equal Validator

I've added an "equal" validator, which basically validates that a field has the same value as another field. (like "confirm email" or "confirm password") I've added docs and a test as well.

The biggest change was that I allowed `Field` to accept the parent `Validator` as a param. I was wanting to get a reference to `Validator#form` so I didn't need to use `closest` to traverse up and back down. It made sense to just pass 1 argument and extrapolate from there.

Also updated the history to be a bit more accurate.

Lastly, there is 1 failing test, but it looks like it is for a feature that was never coded? (async validators, that is)