System Message Styles

## Style Updates
Vastly simplifies our visual treatment of system messaging as per the mocks.

This also abstracts the "close" styles into a separate mixin and applies those styles to the modal and messages pattern.

## Functionality Updates
Adds a new JavaScript function to find all system messages, add a close button and toggle a jQuery `slideUp` animation on `click`.

## Result
![messages](https://f.cloud.github.com/assets/1479130/2415499/5bb4f10e-aaf6-11e3-8171-678af128c715.png)

Dependent on DoSomething/paraneue#45
Resolves DoSomething/dosomething#971