Idea for improving firebase elements

`location="https://{{globals.firebase}}.firebaseio.com/...` still seems like a rather "crude" approach

Since all firebase endpoints have this form, it still seems a bit superfluous. Would be better as `location="{{globals.firebase.endpoint}}`. Might as well extract it into a separate custom element more along: `<firebase-model app="{{globals.firebase.account}}` path="..." />` 
