Added support for exporting Geometry::Square to SketchUp
Added support for Model class attributes

For now, any attribute defined with a default value automatically becomes a class attribute as well as an instance attribute.
Fixed read-only attributes for Model
Updated for Sketch v0.3
Optionalized the name argument to extrusion, model and sketch

If no name is passed, no global constant is set. The new subclass is returned either way.
Refactored the Model, Extrusion, and Sketch, builders

Now the subclasses are built directly instead of building an instance and then moving everything to a subclass. This also fixed the Extrusion attribute problem, and made an Extrusion subclass's sketch a subclass of Sketch.
Added support for Units proxy operators to SketchUp exporting
Fixed SketchUp exporting of sketches with nested groups
Updated for changes made to the sketch gem after v0.3
Extrusion builder must provide a length attribute to the DSL
Added support for SketchUp export of Geometry::Annulus
Added support for SketchUp’s Group class

Now Model::Group is exported as a Group, not a component. This significantly cleans up the Outliner window in SU, and makes it easier to select nested elements.
Allow Engineering::DSL to be called from within module definitions

This allows users to organize model-subclasses into modules…

module Fasteners
    model :Bolt do; end;
end
Extend Model::DSL to support attributes

Model v0.2 dropped support for attributes
Require the latest versions of the dependencies
Require Ruby 2.0 or later
Configured for Travis CI with rubygems releases
Fixed a test