Move model builder into spec/support

it's intended only for use within specs, not by clients of the gem
Refactor ModelBuilder for use as rspec helper

Make it a stand-alone class and define a focused DSL module which can be
mixed into Rspec scenarios (a single `#build_flow` method).
Update specs to use new model builder
Remove unnecessary conditional
Move definition of `otherwise` predicate

Better off being part of the default state