Add CodecJson and corresponding law and convenience functions.

Fixes #18.
Fixes #41. (By exporting law on CodecJson)

After this commit there are some nice conveniences for
declaring Encode and Decode pairs.

For example:

  case class Fred(name: String, age: Int)

  // safe

  val codec = codec2(Fred.apply, fred => (fred.name, fred.age))(name, age)

  // or utilizing generated unapply (partial function, but not really for case classes)

  val codec = casecodec2(Fred.apply, Fred.unapply)(name, age)