A simple caching output controller for Savant

Savant output can be cached for faster rendering. PEAR2_Templates_Savant_Turbo
allows rendered content to be cached, and reused for subsequent requests.
For basic use, instead of creating a new `PEAR2\Templates\Savant\Main` object,
create a `PEAR2\Templates\Savant\Turbo` object. In models you'd like to have
cached, implement the `CacheableInterface`.