All accessors now take a `root` arg, the root of the media tree.
Alternatively `root` can be a custom MediaFile subclass, which should allow
neat tricks like:
- Checking some kind of manifest to prevent stat() calls
- Custom properties of the file objects (e.g. for HTML <img> tags)
- Downloading the media on demand
Tests assume media is at pokedex/data/media, skip otherwise.
