[Tilecache] everyblock use of tilecache & openlayers

[Artem Pavlenko]

Mapnik supports palette based output for png (svn trunk). All you  
need to do is to specify : 'png256' as image format.
See http://trac.mapnik.org/browser/trunk/demo/python/rundemo.py for  
usage.

It works very well in 99.9% cases and if not, please report to  
trac.mapnik.org.  I'm planning to add possibility to specify  
important colours to ensure they're present in final palette.

Cheers
Artem