Added a LDM class where the load()'ed value depends on another model.
Added behaviors to hide/enable components based on a boolean model.
Added a behavior to hide a component when the object of the component's
model is empty or null.
Added a couple of dependent models that are useful for composing models
together.
Merge pull request #357 from ageery/master

Models that depend on other models and behaviors for component visibility/enabling based on models