Fix for collections that are initialized with existing Firebase references

Prevents premature event handling when using an existing Firebase reference by deferring until after the parent constructor is initialized.
Merge pull request #5 from leisms/master

Fix for undefined property error when using existing Firebase references
Create Backbone.Firebase.Model and allow firebase ref arguments to be a function.
Implement model destroy() and put _log() back in
Add naive success callback for destroy()
Trigger sync event when Firebase.Model is updated by Firebase
Log errors in model set and destroy, and prevent initial model change event after first sync
Enable model unset() and re-enable the change even trigger on model update from Firebase
Use a simpler unset() method by looking at model.changed
Handle cases of null set() for model and add support for unsetting attributes automatically if deleted on Firebase
Prevent undefined and null logs
Fix error when loading from nonexistent firebase ref
Merge https://github.com/leisms/backfire into gh-pages