Fix #2470 Do not mutate options in _prepareModel
shorter defaults in model ctor, shorter extends in history
Merge pull request #2795 from akre54/shorter-defaults

Shorter defaults in model ctor
Merge pull request #2783 from caseywebdev/gh-2470

Fix #2470 Do not mutate options in _prepareModel
Test for #2666.
Fixes #2428 -- set now returns added or merged models, so does add, so does push, so does unshift.
Fixes #2401 – Adds mission statement-ish to introduction in the docs.
Firming up returning models from add and remove, plus tests.
Mild DRY-ing, and fixing logic error.
Backbone.js 1.1.0
Fixes weird bug in minified version of code - only in Chrome 28-29