remove duplicate Firebase constructors (re-use references)

remove duplicate Firebase constructors (re-use references)
add missing urlError method
save the original Backbone.sync reference as sync_AJAX
updated documentation

(these are the promised checkins; I've tested them and all working; I'll send you an example that proofs all the changes; you can decide if it's appropriate to include in the project or keep in a separate GitHub (it's more of a boilerplate than proof)

Let me know if you have questions or criticisms! I'll be happy the updates to your vision.