Bump to v0.3.0
Add reference property
Add ElementProperty, rename property files
Better handling of indexable properties
Unique properties are now automatically indexed
Rename name to propertyName
Add single and multi reference property classes
Model properties are now defined in the model.prototype

- Schema references properties as selfProperties or refProperties
- properties attach themselves to model prototype upon model compilation
- models can now be instantiated with default properties
Fix handling of null properties when adding a Model
Fix property values being shared across all model instances
Solve merge conflicts when merging branch v0.3.0 into patch-grex-0.5.x