Can't do Arel operations with nodes

[rhymes]

Hello,

thank you for writing a library to simplify the syntactic sugar on using Arel nodes to address JSON fields.
There's a problem with your approach though: they don't support operations.

For example you can't do:

node = Arel::Nodes::JsonHashDoubleArrow.new(arel_node, Arel::Nodes.build_quoted("{json_path}"))
node.eq(value)

This will raise an exception because JsonHashDoubleArrow inherits from Arel::Nodes::Binary but does not include any of the operations, see for example how the InfixOperation is implemented: https://github.com/rails/arel/blob/master/lib/arel/nodes/infix_operation.rb

As you can see here https://github.com/rails/arel/blob/master/lib/arel/nodes/infix_operation.rb#L6 they include all the operations you can do on nodes, but your code just inherits from Binary: https://github.com/guyboertje/arel-pg-json/blob/master/lib/arel/nodes/pg_json.rb

[guyboertje]

I did not need to do operations on it so I did not see the need. What you see here is a MVP.

Sadly, (for this library) I do not use Rails and Postgres anymore and my days and nights are filled with LogStash code and plugin code, so I am not really able to give this library any attention.

Please provide a PR with tests and I will review and merge it.