comparator fix

Small fix for https://github.com/firebase/backbonefire/issues/121. Moved the comparator so it can be overwritten by extending Firebase.Collection. 

I also noticed a couple `apply(this, [ array ], options)` happening, which means the options get silently ignored. For consistency i changed them all to `#call`. 

I think ordering by priority or child should be addressed in a new issue / pr. Personally I forgot that most apps should sort on the client, so it should probably be added to the docs to discourage premature sort indexing.