ResourceStore API and file integration (GEOS-6311)

Introduce ResourceStore API with initial FileSystemResourceStore and DataDirectoryResourceStore implementations.

Update GeoServerResourceLoader, GeoServerDataDirectory, GeoServerSecurityManager and logging to use resourceStore from application context.

References:

* http://jira.codehaus.org/browse/GEOS-6311
* http://geoserver.org/display/GEOS/GSIP+106+-+Managed+File+API