Allow Collection.at to accept negative indexes

This allows you to do things like

```javascript
mycollection.at(-2) // get second to last, etc.
```
Add test for negative indexes in `at` method
Set correct number of assertions
Update README to better match backbonejs.org 

line 14 readme missing the word "with":
Backbone supplies structure to JavaScript-heavy applications by providing models key-value binding and custom events, collections...
--> on backbonejs.org reads as -->
Backbone.js gives structure to web applications by providing models with key-value binding and custom events, collections...

Also bolded models, views and collections just as they are on backbonejs.org
Merge pull request #3281 from jisaacks/master

Allow Collection.at to accept negative indexes
removed the strong tags, but kept the "with" on 14
Merge pull request #3318 from NickStefan/patch-1

Update README to better match backbonejs.org