Introduce ResourceStore API with initial FileSystemResourceStore and DataDirectoryResourceStore implementations.
Update GeoServerResourceLoader, GeoServerDataDirectory, GeoServerSecurityManager and logging to use resourceStore from application context.
Ensure ResourceStore does not return null and pull request feedback

Change Resources.EMPTY to ResourceStore.EMPTY

Update ResourceStore javadocs so it is clear null is never returned (IllegalArgumentException used for invalid path)

Update utility methods in Resources to throw NullPointerException.

Double check consist resource and directory local variable names.
Merge pull request #531 from boundlessgeo/resource_store1

ResourceStore API and file integration (GEOS-6311)