Add serve_files to testem.

Listing the files to serve will cause testem to not loop testing due to
files created by test/before (which execs the convert script).
No need to retest that Diskette exists.
Add Diskette.fallback()

fallback() sets the baseUrl that files will be loaded from relatively.
fallback() is an alternative to calling config(). Both should not be
called for the same Diskette.