Improve the process of clearing the cache.

Each cache type can consist of one or more cache tags. This patch uses the cache tags
that have been configured in magento for each cache segment instead of using a single
hardcoded 'config' tag.

This touches on issue #19, but doesn't resolve it completely yet.
Add layout cache service and remove unused namespaces
Merge pull request #23 from Vinai/improve-cache-service

Improve the process of clearing the cache.