Add gateway

* Update schema to use a thread safe Hash.
* Add #[] and #[]= methods to the adapter api to allow getting and
  setting the named relations.
* Add initial #update method to the relation/gateway api. Will need
  to add Axiom::Relation::Operation::Update as well as base #update
  method to Axiom relations matching this api.
Add UnknownRelationError exception
Merge pull request #1 from dkubb/gateway

Add a gateway