Fix #69 Workaround RubyMotion dependency detection

By defining a singleton method directly on Teacup, rather than
opening the Teacup module to define it within, RubyMotion's
dependency detection is prevented from diagnosing a dependency
cycle between layout.rb and ui_view_controller.rb.

See issue #69.
Merge pull request #70 from kastiglione/patch-1

Fix #69 Workaround RubyMotion dependency detection