The ExternalDataAsset class defines the properties for an external data source used at runtime by Degrafa.
You can use an ExternalDataAsset object in actionscript - it may be useful to
set up preloading, for example, but in mxml use of an ExternalDataAsset is already encapsulated into
the Degrafa class that uses the content (when it is ready) by virtue of its source property assignment.
The data content provided by an ExternalDataAsset will only be available once the asset from the external url has
loaded.
