Stretch background width to fit for wide screens

This pull request ensures that the background image found on the Home/About section fills wide screens (those wider than the 2000px source image).

On screens wider than 2000px, the Home/About page currently looks like this (notice the blank areas at the left and right sides of the window):
![screen shot 2014-05-23 at 17 14 34](https://cloud.githubusercontent.com/assets/250934/3071973/b6b970a6-e2bf-11e3-9884-fc9502ba638d.png)

With the added media query and background-size property found in this pull request, the background image stretches horizontally to fill the width of the screen as seen here:
![screen shot 2014-05-23 at 17 14 21](https://cloud.githubusercontent.com/assets/250934/3071999/fb019612-e2bf-11e3-9a87-40c973514d4c.png)

The layout is unaffected for browser windows under 2000px in width.