bind memoize cache to object instead of function to limit cache to object lifetime

fixes memory leak when configurations are reloaded inside mapproxy (closes #84)
use alpha_composite by default when available
Documentation about undocumented feature referenced in https://github.com/mapproxy/mapproxy/issues/33
Merge pull request #85 from IvanSanchez/master

Documentation for mapnik source: transparent=true
Fixed markup