bump to 0.9.0 with breaking rename changes in experimental features via mapnik-reference
finish update to use mapnik-reference 0.4.0
remove unused sys require
Rename sys to util to fix last commit
Throw an exception when an unsupported mapnik version is requested
Have render() catch Reference errors by invoking the user callback
Have setVersion return a bool, rather than throwing an exception
Merge pull request #170 from strk/throw_on_unsupported_mapnik_version

Error out on unsupported mapnik version
Fix code style.
Make more types identify themselves with is, don't tolerate
incorrectly-cased functions.
adapt to Mapnik 2.1 syntax for new features that are settling out - specifically now using commas to separate chained transforms or image-filters and using () in agg-stack-blur image filter - refs #171 and #140
use latest mapnik-reference
Merge pull request #173 from mapbox/fix-mapnik-functions

Fix mapnik functions
Remove variable is for the time being
Update mocha version and use spec reporter
Remove anonymous type. Fixes #148
bump to 0.9.1
use latest mapnik-reference
Add unsigned support and test code
Point to numbertypes branch of mapnik-reference