Add support for more generic json/jsonb criteria

[Sabst]

This is a possible approach to make criteria more flexible.
Another syntax could group the 3 first items together ("data->>name" instead of "data", "->>", "name"). I don't think this will make the approach less generic but will change a bit more PgJsonExpression.java (I was unsure about the findColumns call).

This relates to #56

Note: I also tried to see if that helped for #91 but could not find an answer: maybe a projection using PG json functions is needed...

[coveralls]

Coverage increased (+0.1%) to 80.495% when pulling 92cf99b on Sabst:feature/56/json_operators into 5db768a on kaleidos:grails-2.x.

[ilopmar]

I've been reviewing this and ported it to Grails 3. I'll merge it tomorrow and release a new version for Grails 2 and another one for Grails 3 with this changes.

Thank you very much for your contribution!

[ilopmar]

Published version 4.6.2 for Grails 2.

[ilopmar]

And also release 4.6.8 for Grails 3.