First (logical) commit of FlowModelBuilder extension

Draft implementation of functionality to enable new flow model builders
to be registered and added to an application:

 * Created a 2.4 version of the webflow config schema with the inclusion
   of a flow-model-builders namespace element
 * Refactored general merging and resource handling
   logic from XmlFlowModelBuilder into 
   AbstractResourceBackedFlowModelBuilder whilst preserving backwards
   compatibility.
 * New FlowModelBuilder implementations registered by class and not
   as Spring-managed beans (open question: any value in having them
   Spring-managed?). 
 * Added 'lmx' as a test resource to the pom.xml

Note: included are a couple of modifications that I had to make to the POM in order for me to build the project.