Add support for PostGIS Raster

[strk]

Mapnik already supports PostGIS Rasters via the GDAL plugin.
Another PostGIS-Raster plugin is in the make (see mapnik/mapnik#1660).

In any case, for our tiler to support postgis rasters we'll need to extend the MapConfig document to allow for specifying a field name and type when it comes to creating a map.

Then, the tiler would need to create the appropriate Mapnik XML based on that MapConfig (would likely involve enhancing grainstore as well).

[strk]

Also needs CartoDB/grainstore#93

[rochoa]

I'm closing this issue as the PR was already merged.