Model#push can accept a class to instantiate

If the first argument is a Class, it will call new on that class and pass it the rest of the argument. If the first argument is an instance derived from Model, it will convert the remaining parameters to a Transformation, apply the transformation to the instance, and then push it.
Updates for the latest Geometry changes

Rectangle#new requires named arguments
Tell bundler to get the sketch gem from GitHub
Require sketch v0.1
Configured for Travis-CI