Add option to inherit files from a different parent (not default necessarily)

- Support specifying "parent-child" skin relation by specifying the name of the parent in [skin name]/parent.txt
- Simplify the loading of templates and remove caching (cached.Loader takes care of that better)
- Add cache for media lookup (to avoid accessing the filesystem on every template render)

This is needed to support templates derived from BinaryRage's infinite scrolling template (which I'm adding later).