Updating rotating sponsor logos on home page

Sponsors are managed as distinct *post* types, each with their own logo. and expiry date.


 * Current sponsors are displayed in rotation on the bottom of each page, including the home page.
 * Current sponsors are displayed with their logo, according to sponsor level, on the http://osgeo.org/sponsors/ page
 * Prior sponsors, are thanked at the bottom of the http://osgeo.org/sponsors/ page

To manage sponsorship your OSGeo userid should have *Sponsorship Editor* role.