Adds configuration for centralized cache. Adds shell script to bust cache.
Adds Twig options to set cache path for TwigServiceProvider.
Configures HTML Purifier cache directory, if enabled.

- Adds a few helper methods to Bootstrap to resolve paths.
Adjusts htmlpurifier cache directory permissions to 0755.
Merge pull request #133 from mdwheele/feature-single-cache-directory

Progress towards implementation of combined cache directory.