Throw error if the factory being build is not defined.

The current exception is:
     TypeError: Cannot call method 'build' of undefined
which is not very informative.
Merge remote-tracking branch 'origin/master' into onetom-report_undefined_factory

* origin/master:
  Add instructions for contributing
  Removing mocha from devDependencies
  tweaks
  Switch to a JavaScript test runner
  Switch to MIT license
remove commented out require