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
Add docs and changelog for archive import
Merge branch 'import-zip'

Closes #156