Dynamic layers via Mapnik XML style stored only in memory

This patch adds support for using "in memory" Mapnik XML styles instead of a filename.

Mapnik provider 'mapfile' property accepts string starting '<?xml ..' and dynamically loads it via mapnik.load_map_from_string() call.

Technically, this patch allows to create dynamically filtered layers (via modified SQL subqueries) or layers with dynamic colours/styling specified in the layer name via a custom WSGIServer similar to 
https://github.com/migurski/TileStache/blob/master/TileStache/Goodies/ExternalConfigServer.py