Add support for document types (List and Map), Boolean, and Null

[willwhite]

Adds dependency on part of dynamodb-doc, which does two-way, recursive transforms between native JavaScript objects and the DynamoDB wire format.

• Add support for document types (List and Map), Boolean, and Null
• Native JavaScript arrays will now transform into lists instead of sets
• Add dyno.createSet(), which constructs sets (of number, string, or binary
  type) that transform to the DynamoDB wire format correctly.
• Drop support for using a wire-formatted object as input.

Fixes #41

/cc @rclark @jakepruitt @emilymcafee