Improve management of external graphics

Our implementation of VariableOnlineResource was wrong. We were not able to manage cases where the drawn image was dependant upon some input parameter. That means it was not possible to use Categorize or Recode instances to manage the URl of the external graphic.
I try to improve that by managing asking the parameter the URL according to the input data each time a feature is requested. I maintain simple caches of data (images and boundaries) in order to be able not to query the resource provider for each feature. This caches are purged whenever the underlying parameter that manages the UI changes.

Related to #425.

You can find a recoded ExternalGraphic here : https://gist.github.com/agueganno/5756455