rule dependencies

The old library had the ability to make rules dependent on others, i.e. re-evaluate a binding when other bindings are updated; now, this won't work:

``` html

```

Assume `passwordConfirm` must be equal to `password` (supported for example in `aurelia-validatejs` with `@equality`) the `passwordConfirm & validate` binding should trigger also when `password` changes, but the binding behavior doesn't know about the implementation specifics so it won't update/render when `password` is changed.

Reproduce the password and passwordConfirm bindings from the issue, then locate the validation binding behavior and how it tracks dependent bindings. Confirm the fix by showing that changing password re-evaluates and renders the passwordConfirm validation state.

Written by the indexing model from the issue text.