- added "cache" accessor to Template, Namespace.

  e.g.  ${local.cache.get('somekey')} or
  template.cache.invalidate_body()

- the Cache object now supports invalidate_def(name),
  invalidate_body(), invalidate_closure(name),
  invalidate(key), which will remove the given key
  from the cache, if it exists.  The cache arguments
  (i.e. storage type) are derived from whatever has
  been already persisted for that template.
  [ticket:92]