v2.2.0 release

Work in progress. Before merging we need:

 - [x] - source tarball posted
 - [x] - blog announcement (@artemp)
 - [x] - fixed links for downloads
 - [x] - update https://github.com/mapnik/mapnik/wiki/MapnikReleases
 - [x] - update https://github.com/mapnik/mapnik/wiki/Release2.2.0
 - [x] - double check https://github.com/mapnik/mapnik/wiki/MapnikReleaseSteps for steps we missed