Update only attributes that have changed.

Backbone models have a changedAttributes() method, that returns only the attributes that have changed since last set.

http://backbonejs.org/#Model-changedAttributes
Use underscore's bind instead of relying on native bind to be there.
Merge pull request #51 from hijonathan/bind-fix

Use underscore's bind instead of relying on native bind
Merge pull request #50 from hijonathan/gh-pages

Apply model defaults
Leave defaults set on Backbone.Firebase.Model before first sync
Merge pull request #48 from mihar/gh-pages

Update only attributes that have changed.
Handle model.destroy() on a Backbone.Firebase.Collection, closes #49.
Implement Collection.reset() method, fixes #43.
Move todos example to different directory
Add flatdoc page for Backfire
Tweak repo URL in README
Add Grunt and bower integrations
Add ref() to retrieve Firebase ref from regular models/collections, fixes #16
Add event for initial sync, closes #13
Add documentation on Backfire, closes #44, #26
Make Backbone.Firebase.Collection.create return newly created model, closes #15
Keyword `new` interpreted specially, remove from docs
Add bower install info
Ignore bower, obvi.
Add underscore as a direct dependency.