Various doc fixes.
json error handler : use MAPNIK_LOG_ERROR and better message
avoid allocating potentially large std::string in error_handler
use error_handler across json parsers
position_grammar - re-order rules to allow expectation operators
Merge pull request #2431 from joto/doc-fixes

Various doc fixes.
c++11 - use auto
point postgis-inline.xml at template_postgis
only accept JOBS via make setting and default to 1 - closes #2430
add visual test for expressions function length(<expr>)
remove unneeded include
Remove unused unifont
actually fix make clean first run - refs #2112
back to 2 JOBS
ensure boost system is linked after thread - closes #2436
postgis tests - skip if createdb not found (TODO: check return code)
skip load/save map tests when postgis is not running
add to string impl for length func - closes #2437
skip postgis tests if postgres not running
replace tabs with spaces - amends c322655c