Pricing Table Layout

This PR adds a new **Pricing Table** layout to the ever-growing list of Pure layouts. It's an enhancement of the original pull request that was submitted by @rashadrussell a few weeks ago. The enhancement includes:

* Prettify with CSS
* Make HTML more aligned with SMACSS

## How's it look?

Pretty nice, IMO.
![](http://f.cl.ly/items/391X2K3y193X0p170310/pricing@2x.jpg)

## Why a pricing table?

* Pretty common page for a lot of product-based sites. 
* Goes well with the marketing layout
* Good example of how to make pricing tables based on grids


## Tested

* IE7+
* Latest Chrome
* Latest FF
* Latest Safari
* iOS6 on my iPhone and iPad
* Android 4.0.1

The grid in IE7 has issues, because of the sub-pixel rendering of IE7. This is a known issue and I have sent a PR to fix it [here](https://github.com/yui/pure/pull/147). I have tested the layout and it's fine once this fix is applied.

It looks good on phones, but I think the menu will look a lot better once we can leverage the `.pure-menu-responsive` class, so I'm looking forward to updating this layout once that becomes available.