+ fix writing to std::stringstream

* if seeking pass the end  - grow buffer (stringstream!)
* seekp beyond current buffer size doesn't set failbit (clang/libcxx)
Merge branch 'master' of https://github.com/mapnik/mapnik
Merge branch 'master' of https://github.com/mapnik/mapnik
Merge branch 'master' of https://github.com/mapnik/mapnik
Merge branch 'master' of https://github.com/mapnik/mapnik
remove uneeded exception translator for mapnik::config_error - works around #1847
+ move to QT 5.x (requirement!)
scons: disable cairo support at configure time unless cairo reports freetype support - closes #1842
Merge branch 'master' of https://github.com/mapnik/mapnik
further improve cairo freetype support checking - refs #1842
Merge branch 'master' of https://github.com/mapnik/mapnik
remove debug output
Merge branch 'master' of https://github.com/mapnik/mapnik
only run cairo visual rendering tests if cairo backend is available
update expected grid test
Merge branch 'master' of https://github.com/mapnik/mapnik
* c++ style comments, pls!
* formatting
+ add python_optional<float> specialization
+ use mapnik::noncopyable
finish cleanup after geos plugin removal - refs #1809