Remove unnecessary prefix from documented type
Add spec for Axiom::Adapter::Memory#read

* Change Adapter#read interface to yield a Tuple.

  This is a relatively large API change, one that will be pushed out
  to other adapters and a base adapter if successful.

  Inside Axiom, we have attempted to reduce the number of primitives
  we expose at the boundaries of each class. Internally, we may
  sometimes encapsulate a Hash or an Array, but we attempt to not
  have as much of a primitive obsession.

  Inside Axiom we already have an object to represent a row from the
  adapter. There is no need to yield an Array in #read.
Add spec for Axiom::Adapter::Memory#insert
Add spec for Axiom::Adapter::Memory#update
Add spec for Axiom::Adapter::Memory#delete