Front page ad for SOTM 2013

This pull request adds an ad to [the State of the Map 2013 USA site](http://stateofthemap.us/). It looks like this:

![Screen Shot 2013-03-14 at 4 15 07 PM](https://f.cloud.github.com/assets/108094/260977/f462aaa4-8ce3-11e2-90ff-32076e9acd25.png)

We're kicking up communications around the conference and have opened the call for proposals today. Early next week we will open registration. It's become important to drive more people to the conference site so we'd like to get an ad up on osm.org, ideally before the weekend,