Fix: utils.debounce timing corner case.

Invocations of the debounced method between when runNow is scheduled
and when it actually runs will schedule additional runNow invocations
to be executed in rapid succession.
Merge pull request #486 from jamestalmage/fix-debounce-timing-corner-case

Fix: utils.debounce timing corner case.
Return uid of created user
Make $createuser() resolve entire user object, not just uid
Merge pull request #492 from firebase/jw-create-user-uid

Return uid of created user
Merged with master branch