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.