Make CacheHelperExtensions public so we can call CachedPartialView and ClearPartialViewCache

In order to allow extensions to the logic and usage of the CachedPartial HtmlHelper, expose CachedPartialView and ClearPartialViewCache by making the wrapping static class public.
Merge pull request #144 from IDisposable/patch-2

Make CacheHelperExtensions public so we can call CachedPartialView and C...