Add :map field type

[andrykonchin]

Changed:

• Added map type

There are already in use array type, that matches the List DynamoDB type, and raw type, that matches automatically any supported DynamoDB type (including List and Map). But there is no dedicated type for Map which forces using raw type in order to store Hash/JSON-like data. It may confuse users (and actually there was created issue with such request).

Example:

class User
  include Dynamoid::Document
  field :settings, :map
end

User.create(settings: {foo: {bar: ['baz]'}})