a couple of library cache clearing methods and moves them to CacheHelperExtensions. Fixes #U4-1653 for 6.1 and updates ExamineEvents to
ensure that Examine re-indexes media on all of the correct events (including Trashed). Changes for Caching are being done to begin work with
CD-148
Created new CacheHelper which replaces the old Cache class and is accessible via the UmbracoContext.
Added new CachedPartial extension method to be able to cache the output of your partials (also by page and by member)
Added mew CacheHelperApplicationEventListener in order to automatically clear the partials cache when content is published, media or members is saved.
