Added traits class to domain.
Merge pull request #2 from IDgis/model-traits

Added traits class to domain.
LayerTypes and ServiceTypes are wrapped in Traits objects in the
registry.
Traits for ServiceTypes and LayerTypes can be configured in the
application context.
Merge pull request #3 from IDgis/layer-service-traits

Layer service traits
Merge remote-tracking branch 'IDgis/master' into client-mapmodel
Implemented linkedCopy: copy-on-write copies of stateful objects and
arrays that keep a link to the original.