Add support for Postgresql datatype jsonb

Hello,

While working on a [lightning talk](https://gist.github.com/pcreux/cd6efb47f8bc439054bb) on the [postgresql datatype jsonb](http://www.postgresql.org/docs/9.4/static/datatype-json.html) I added support for jsonb to active record.

jsonb is available in Postgresql 9.4 beta that has been released on May 15, 2014.


Dont test them separately. Test that the type lookup is an instance of `OID::JSON`, and that `type` returns `:jsonb`