Add Button Groups to Pure

This PR adds horizontal button groups to pure, thereby fixing https://github.com/yui/pure/issues/126. 

## Why we need button groups
I think button groups are a good way of grouping multiple buttons together to give them some sense of context. This can be useful in a variety of websites. 

## How it works
The button groups are wrapped in an element with the `.pure-button-group` classname. This element inherits the styles from `.pure-g` to collapse the whitespace within it. The `.pure-button` elements within this reset the letter and word-spacing, similar to `.pure-u`. 

## How it looks

![button groups](http://f.cl.ly/items/3P0m330c0p04241l170S/Screen%20Shot%202013-07-23%20at%2011.33.40%20AM.png)

## Test Coverage

* IE7+
* Chrome
* Safari
* FF
* iOS6
* Android 4.0.x.