Add tests for the `importadded` plugin

The tests verifies that the `importadded` plugin updates added dates and
mtimes when importing:

* an album
* an album with file mtime preservation on
* singletons
* singletons with file mtime preservation on

It also verifies the the plugin doesn't update added dates when
re-importing:

* an album
* singletons
`beet stats` output clarification (#1052)

Explicitly say that size calculation is approximate.
Merge pull request #1054 from silb/importadded-tests

Add tests for the `importadded` plugin
Tiny formatting tweaks (#1054)