Implement AttributeDictionary class

[noelwarren]

Details here

https://ruby.sketchup.com/Sketchup/AttributeDictionary.html

Please note it includes enumerable and inherits from entity

Also note that the values stored in an attribute dictionary are not any object:

• Key can only be String:
• Value can be one of Boolean, Integer, Float, Length, nil, String, Time, Array, Geom::Point3d, Geom::Vector3d