[openstreetmap-website] SotM Banners on the Front Page (#268)

[John Firebaugh]

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.)
You can merge this Pull Request by running:

  git pull https://github.com/osmlab/openstreetmap-website sotm-ads

Or you can view, comment on it, or merge it online at:

  https://github.com/openstreetmap/openstreetmap-website/pull/268

-- Commit Summary --

  * Remove commented out donation section
  * Add SotM ads

-- File Changes --

    A app/assets/images/sotm-birmingham-ad.png (0)
    A app/assets/images/sotm-us-ad.png (0)
    M app/assets/stylesheets/common.css.scss (23)
    M app/views/layouts/site.html.erb (20)

-- Patch Links --

https://github.com/openstreetmap/openstreetmap-website/pull/268.patch
https://github.com/openstreetmap/openstreetmap-website/pull/268.diff

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