PR: Extensions, tests and namespaces

Hi!

I`ve added support for custom extensions and tests. 
Also I`ve completely rewrited CFS loader for allowing namespaces (this is a feature of Twig, see here http://twig.sensiolabs.org/doc/api.html). It reduces filesystem scans and stores actual list of twig-related paths/namespaces in Kohana application/cache.

This version is in production now, it`s stable and, as I see in site stats, has no performance leaks.

Best wishes,
Denis.