Extract check for thrird party programs in tests
Import zip archives

`beet import archive.zip` extracts the archive to a temporary directory and
imports the content.

The code is very hacky. To make it cleaner the `importer` module needs some
refactoring. One thing the code hints at is extending the `ImportTask` class.
Import tar archives

Also refactors the importer code to make better use of ArchiveImportTask.
Import rar archives
Catch all archive extract errors and skip tests without unrar