Toast aggregation

https://www.pivotaltracker.com/story/show/81996192

Invoking `notification.error` _(as well as `.notice`, `.success`, ect.)_ with a `category` provided in the options parameter will enable the grouping/aggregating functionality.

Example:
```coffee
notification.error 'The message', category: 'NotFound'
```
Would produce a grouping of notifications that have the same category, but different messages:
![image](https://cloud.githubusercontent.com/assets/2935356/4925163/0234a83e-6529-11e4-9512-3b7c57a2e6cf.png)

An aggregation would occur when notifications have the same category, message, and type:
![image](https://cloud.githubusercontent.com/assets/2935356/4925235/6d6d09fc-6529-11e4-9f80-0a45b274440a.png)

**All existing notifications still work. This is a non-breaking change.**