Album queries should automatically use albumartist instead of artist

When typing queries on the command line, it's sometimes a pain to remember to use `albumartist` instead of `artist`. Beets should special-case this field and automatically use `albumartist` when the user types `artist`.

This should "just work", for example:

```
$ beet ls -a artist:beatles
```

Perhaps this should also happen for templates (`$artist` falls back to `$albumartist`).

(This came up in #307.)