Add a Blog Layout

This pull request adds a Blog layout as a 4th layout. Tested in IE7+, Chrome, FF, Safari, iOS7 and Android 4.0.x.

Here's how it looks:

![blog](http://f.cl.ly/items/292k2B2a0x0h1c0r3d0b/Screen%20Shot%202013-06-13%20at%204.26.36%20PM.png)

The left-sidebar pops up to the top on smaller screens.