Migrate GeoServerPersister to GeoServerResourceLoader (GSIP-106)

Continuing from #568

Modified GeoServerPersister to use Resources rather than Files and provided some Resource oriented methods on GeoServerDataDirectory to support this while deprecating its File oriented methods.

Things to watch for while reviewing:

Graphics used by SLDs are by far the most complicated aspect of this.  It needs to copy graphics that are in the style directory with the SLD to the new style directory, and verify that a real file system file exists for such graphics when passing a parsed style out.  It always exists at present, but future implementations may need this step to "unpack" resources to the local file system.

For background see http://geoserver.org/display/GEOS/GSIP+106+-+ResourceStore+API

Specifically the notes http://geoserver.org/display/GEOS/Resource+API+Transition+Plan
I've put together both file name and blocking based implementations. 

https://github.com/smithkm/geoserver/pull/2

I'm inclined to go with the unique ID solution as it's simpler and less prone to deadlock, etc.