Schema and Type declaration support for ORM

[noppoMan]

This is new function for building more typesafe app with npdynamodb.ORM.
Here is desired syntax.

var Chat = npdynamodb.define('chats', {
  npdynamodb: npd,

  schema: new Schema(function(s){
    s.string("id").hashKey();
    s.number("timestamp").rangeKey().default(s.fn.NOW);
    s.string("message");
    s.map("user", [s.String, s.Any]);
  })
});