Add fallback to parsing json string to `PGStore.get`

Prior to postgresql 9.2 it doesn't have a json datatype. Let's give
people not having the luxury to use all the finest all the time a
chance to use this module.
Remove hardcoded schema from sql

Schema shouldn't be fixed by the module imo. It's configurable from
db by user bases and is always set. If one is using credentials on
connection he or she is already making clear of the "playground".
Make it more failsafe

If for whatever reason the JSON is unparsable don't die but destroy
the corrupted row.
Merge pull request #2 from Rauno56/master

Add fallback to parsing json string to `PGStore.get`