Remove generated properties dependecies on `data` attribute, so when manually setting a property
it doesn't fire propertyDidChange twice.
Verifying that setting an attribute fires only one observer.
Removes circular dependency on the internal property `data.`

We want to be able to read / write through to the data property.
We also want to know when new data comes in from the API.
Merge pull request #33 from zendesk/igor/no_data_dependency

[WIP] `propertyDidChange` firing twice every time a property is set