cls_ft_cache.php: Template engine with dynamic blocks and caching.

This class this extends the "ft" class to add caching functionality.

The caching was designed with four points in mind:

1. Speed improvement

2. Ease of use while letting the developer decides when caching is needed on a per file basis.

3. Having an option to set a maximum execution time from which the caching starts to work for each script or page.

4. Having a simple way to expire all cached content when the database or source of content data is updated.

See the source class for more info.