mapnik::value

to_expression_string - add missing specialization
for mapnik::value_integer and remove usage of std::stringstream
remove uneeded python imports
Add test for image.fromstring - refs #1805
commment failing tiff test - refs #1831
Add tests for frombuffer as well - refs #1805
Merge branch 'master' of github.com:mapnik/mapnik into visibility-hidden