anidb: Add shortcut cache querying functions

For instance (*File).Anime() instead of (*File).AID.Anime().
anidb: Add EpisodeString field to File

Pre-formats and pre-processes the EpisodeNumber.
misc: Add (*EpisodeRange).Len()

Use Valid() to check for validity on EpisodeRanges. Remove an unhelpful
panic.
misc: Implement json.Marshaler/Unmarshaler in EpisodeList

Makes for nicer JSON dumps with anidb-like lists instead of unnecessarily
exposing all the internals.
anidb: Support querying for mylist of an Anime

It's not very informative due to AniDB-side restrictions.
anidb: Automatically run USER query after AUTH

This also lets us have a convenient adb.User() that returns the current
user; this only works though if we're already authenticated.