Files are now preloaded

Fixed a bug with fallback files, and changed the syntax slightly. Image
and sound data is preloaded where possible now, which closes #101.
`Battle.stage` has been replaced with `Battle.state` whose purpose has
been changed, and which has been replaced with an object (closes #81).
Restructured Transform

The effects are far-reaching, and I’m still not 100% happy with
Transform—it has too much specific code, but it’s better than it was
before.