changed way to detect missing key since None is already a possible value
Merge pull request #145 from michele-comitini/postgresql_json_bug

bugfix native json fields were not supported correcly
Merge pull request #144 from michele-comitini/dal_sum_expression_fix

changed way to detect missing key since None is already a possible value
removed '[__+-+...+-+__]' from last commit
fixed issue 1600:Centralized migrations log, thanks Alan