Add a call to action for sponsorsips on sponsors page

It'll look like this:

![railsbridge boston 2013-09-04 23-09-43](https://f.cloud.github.com/assets/602470/1085617/9d81a720-15d8-11e3-877a-61266e7458b2.jpg)

Or would it be more appropriate at the bottom of the page?