Improves but does not fix #400 - $bindTo not working with inputs
Workaround for #133 - can't change $priority or $value in $bindTo (they are now stored as _id, _value, and _priority, which trigger updates for $watch)
Fixed #400 - $bindTo not working with inputs
Fixes #133 and #400 in an authoritative and awesome manner.
Fix e2e tests to run with new $bindTo structure.
merge from master
Merge pull request #407 from firebase/kato-403

Optimize our $bindTo code base