Add stub methods for hiding/exposing properties.
Rework method names.

I don't see a real use for incrementally building the hidden field list.
The code is much simpler without it, so I'll leave it off until there is
an actual use case for it.
Add stub test case methods.
Use combined getter/setters for virtual & hidden properties.

The rest of CakePHP uses combined get/set methods, follow that pattern
here as well.

Add test cases as well.
Capitalize JSON.
Merge pull request #2394 from markstory/3.0-entity-hide

3.0 - Allow properties to be hidden and computed properties to be exposed