Provides a version string for a cacheable resource specified by {@link org.apache.wicket.request.resource.caching.IStaticCacheableResource}
the version string will be used as a distinguishing mark when rendering resources
@see org.apache.wicket.request.resource.caching.QueryStringWithVersionResourceCachingStrategy
@see org.apache.wicket.request.resource.caching.FilenameWithVersionResourceCachingStrategy
@author Peter Ertl
@since 1.5