Adopt new alert styles / colors

This converts Asimov alerts to the new alert style currently in handover, and @dannymidnight's full screen viewer. Default alerts now include:

```css
.alert--success
.alert--error
.alert--caution
.alert--info
.alert--admin
```

Examples: 

```css
.alert--success
```

![screenshot 2014-07-16 20 50 47](https://cloud.githubusercontent.com/assets/131831/3597783/a827da7c-0cd7-11e4-8f6c-3b134e8bb60d.png)

```css
.alert--error
```

![screenshot 2014-07-16 20 56 53](https://cloud.githubusercontent.com/assets/131831/3597794/f9453a44-0cd7-11e4-9017-a70497f097d6.png)

```css
.alert--caution
```

![screenshot 2014-07-16 21 01 22](https://cloud.githubusercontent.com/assets/131831/3597831/9192cb2c-0cd8-11e4-98fa-46e52cbe44e6.png)

```css
.alert--info
```

![screenshot 2014-07-16 21 02 25](https://cloud.githubusercontent.com/assets/131831/3597839/b5f34df2-0cd8-11e4-87e5-2b5c07f66253.png)

```css
.alert--admin
```

![screenshot 2014-07-16 21 03 41](https://cloud.githubusercontent.com/assets/131831/3597847/e17cfbe4-0cd8-11e4-8a5b-897c0965cabc.png)

The only thing that makes this a breaking change is a default bottom margin (gutter) provided to alerts.

Depends on https://github.com/99designs/asimov-platform-settings/pull/7/files

/cc @andrewk @xzyfer @dannymidnight @dennisroethig 
They don't seem to.

https://github.com/99designs/contests/blob/master/Gemfile
https://github.com/99designs/swiftly/blob/master/Gemfile

(And an ag shows nothing of interest)