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)
Merge branch 'add_template_inheritance'

Conflicts:
	settings.py