- added "cache_enabled=True" flag to Template,
  TemplateLookup.  Setting this to False causes cache
  operations to "pass through" and execute every time;
  this flag should be integrated in Pylons with its own
  cache_enabled configuration setting.

cache sends in a "defname" parameter so that the cache can map settings to that name, instead of the key.
eliminates the need to store all Value objects which will use up RAM in a dynamic-key scenario.

  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]

an expired section, instead of the original context
passed in [ticket:87]

a cached block would raise an error when called within a
cache-refresh operation that was initiated after the
initiating template had completed rendering.

  directly to the %page or %def, i.e.
  <%def name="foo(x)" cached="True" cache_key="${x}"/>
  [ticket:78]

  caching, combined with an expression-based cache_key,
  combined with the usage of <%namespace import="*"/>

cache function optional