[GSIP-114] PathMapper extension point to control REST file upload locations

Please look at the Proposal on GeoServer http://geoserver.org/display/GEOS/GSIP+114+-+PathMapper+extension+point+to+control+REST+file+upload+locations.

The following pull request contains:

* implementation of the RESTUploadPathMapper interface.
* Modification on the REST API supporting RESTUploadPathMapper class.
* Wicket configuration Panels extending the Global and WorkSpace settings Pages, for configuring the RESTUploadPathMapper parameters.
* A new plugin with a new RESTUploadPathMapper implementation supporting ECQL expressions.
* Documentation pages explaining how to configure the RESTUploadPathMapper parameters and how to implement a RESTUploadPathMapper subclass and its associated wicket page.