Add a couple more float to string conversion tests

See #1811
Merge pull request #1814 from strk/master-more-float-tests

Add a couple more float to string conversion tests
fixup postgis null comparision tests
add missing exception translator for datasource exceptions by catching std::exception - closes #1816
add test for #1816
fix spelling of unknown
also translate std::out_of_bounds => IndexError otherwise new std::exception => runtime catch will override - refs #1816
fix visual test reporting to show test that could not be run rather than just 0
fix handling of null values for feature id in sqlite/postgis input - closes #1817
Merge branch 'master' into image-readers-c++11

Conflicts:
	include/mapnik/factory.hpp
+ use nullptr
+ fix : output file_name
+ png_io: disble miniz for now
+ remove "-ansi" compile flags to avoid clashung with -std=c++11
+ use std::unique_ptr instead of std::auto_ptr (deprecated!)