Allow visitors on the generated models

Based on https://java.net/jira/browse/OGC-55

When working with OGC Filters and such, it can be messy writing business logic to navigate trees and such.
I propose that the models be generated with visitor support. There are some examples of it being used for OGC JAXB Schemas here.
Plugin uses the Apache 2.0 license.
https://code.google.com/p/jaxb-visitor/

Start by reviewing the generated-model process described by the repository and the OGC-55 reference, then compare the proposed approach with the linked jaxb-visitor examples. The work is complete when the generated models provide visitor support for the OGC filter trees and the generated output reflects that behavior.

Written by the indexing model from the issue text.