PostgreSQL jsonb support

Adding support to ActiveRecord for jsonb column type when using Postgresql 9.4+

For details about Json vs Jsonb implementations:

http://www.postgresql.org/docs/9.4/static/datatype-json.html   


For ease of testing, I've setup a Vagrant box with PostgreSQL 9.4 Beta 1 & a Ruby 2.1 environment ready to go if that's your thing:

vagrant init chris-teague/centos-postgresql94-ruby21