Add list placement to text and shield symbolizers from 2.1.0 onwards.

References https://github.com/mapnik/mapnik-reference/issues/45

Enables a CartoCSS workaround - see
 https://github.com/mapbox/carto/issues/238#issuecomment-19673987
Merge pull request #53 from gravitystorm/list-placement

Add list placement to text and shield symbolizers from 2.1.0 onwards.