Added a mechanism that allows developers to add extensions to the code generator.  It works by setting <generatorExtensionClass> to the fully qualified name of a class that implements org.raml.jaxrs.codegen.core.ext.GeneratorExtension.  onCreateResourceInterface is called immediately after generating the Java interface for a resource.  Similarly, onAddResourceMethod is called after generating the Java method that corresponds to a resource action.  Lastly, AddParameterFilter is called when a new Parameter (e.g., URLParameter, QueryParameter, etc). is created.  It allows the method to return false to filter out this parameter.