Progress towards implementation of combined cache directory.

This is some initial work towards centralizing how Twig / HTML Purifier cache is managed and configured.  There is a request / issue for this at #50.  I stress the **initial** part as I have a few questions:

1. Should cache directories be independently configurable or is it reasonable enough to just configure all packages that cache to point to a namespaced directory under a globally configurable root cache directory?  I have implemented the latter, though it needs some cleaning up.
2. Should the cache be enabled by default?
3. Should I be able to enable certain caches while disabling others?
4. Should the application ship with a default cache folder out of the box?
5. Take a look at default permissions for HTML Purifier cache directory. [Documentation](http://htmlpurifier.org/live/INSTALL) says `0755` is appropriate, but that seems a little high. Probably not worth worrying.

In addition to this, there are some considerations to make around easing deployment with regards to expected server write permissions.  The documentation mentions:

>May need to edit directory permissions for some of the vendor packages. (your mileage may vary) NOTE: We're looking at you ezyang htmlpurifier.

Anywho, thoughts?