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().
Update the README to reflect the dependent attribute feature.
Merge pull request #21 from eventualbuddha/dependent-attributes

Add support for dependent attributes and options.