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)
refactored code to match external code example
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
added explicit version option
fixed cmd line args for tests
capabilities printer now works with sublayers;
in addition to the layers, metadata is also parsed
Merge branch 'master' of git://github.com/mapproxy/mapproxy into mp-util_130capabilities