Fix postgresql connection leaks

Return connection to pool on connection error (see #1708)
Fix leaks on persist_connection=false (#1711)

Includes testcase for #1711
Always return a positive number of different pixels.
Update text related reference images.
scons: add HOST option, which if set will skip configure checks - in the future it might do more
Merge branch 'master' of github.com:mapnik/mapnik
fix expected point symbolizer rendering tests after fe526fd3d6dbf
Pylint.
Handle glyph offsets in cairo renderer.
Add option to disable generating new reference images.
add image io tests for proper error behavior on invalid images - refs #1775
fix jpeg/tiff error handling on failed reads - closes #1775
- avoid treating all CPPDEFINES as a whole in scons
- scons help cosmetics
- avoid using double quotes when already escaping as string
- avoid using clog/cerr in sqlite plugin (use mapnik::logging facilities instead)
+ mapnik::projection add operator<<
+ fix assignment operator #1776
Improved error messages.
Show render times for each test.
Fix checks in Map::resize().