SotM Banners on the Front Page

This is a reboot of #217 based on [mailing list discussion](http://lists.openstreetmap.org/pipermail/design/2013-May/000083.html). It shows either a banner for SotM Birmingham or for SotM US, static, 50/50 split. (A version which faded between the two on a 10-second timer was also considered, but found to be visually distracting.)