Test that mtimes are updated when writing files

Follows the discussion in #937 and the wiki [1].

[1]: https://github.com/sampsyo/beets/wiki/Architecture#modification-times-mtime
Combine try_write() and store() as try_sync()

This makes sure we store the updated file `mtime`. By providing the
same interface on `Album` and `item` we can also reduce some code
duplication in the `modify` command.
Merge pull request #958 from geigerzaehler/item-sync

Update database mtimes in modify and write commands