implement 3 ways binding

[pdemilly]

Right now js-data bind method binds the scope to the datastore but not the adapter model which in the case of firebase doesn't make it very sexy

I propose that js-data bind first check if the adapter offers its own bind method and if it does uses it instead.

[jmdobry]

I think the plan here is for js-data-firebase 3.x to add a FireStore component that would extend the DataStore component found in JSData 3.x.