refactor media module, change --env to --root

This commit moves the media scanner out of the db module and into
a new module, media. We also change the --env parameter to --root,
which takes a path to a directory where the default configuration
and the transcoded media cache will live.

groove/shell/interactive_shell.py

@@ -1,7 +1,7 @@
 from slugify import slugify
 
 from groove.db.manager import database_manager
-from groove.db.scanner import MediaScanner
+from groove.media.scanner import MediaScanner
 from groove.shell.base import BasePrompt, command
 from groove.exceptions import InvalidPathError
 from groove import db