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.
Switch to a JavaScript test runner
tweaks
Removing mocha from devDependencies
Add instructions for contributing
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
Add support for dependent attributes and options.

Dependent attributes are attributes with dynamic values which depend on other attributes (or options) to derive their value. Options are like attributes except that they do not appear in the built object and are thus useful for controlling how attributes are built without appearing in the final output.
Generalize the Ember support into any constructor with create().