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
Minor typo fix
Merge branch 'import-zip'

Closes #156
Fix typo in changelog
Merge pull request #691 from camikazegreen/patch-1

Minor typo fix
Add drop option to FtInTitlePlugin
minor comment/doc tweaks for #690

The description of the import command was getting a bit unwieldy. Splitting up
the paragraph helped a little, but the docs could still use some love here.
more readable changelog for 1.3.5
spell check
version bump for 1.3.6
Test mbsync plugin

Exposes bug from #693
Refactor mbsync
Query string for computed fields. Fixes #693
Check for singleton directly
Test albums and singletons simultaneously
query-based fix for #693

This is @geigerzaehler's fix from b868799, just with the typo fixed
(singletons -> singleton).

An alternate (more verbose) fix would have been:

    singletons_query.subqueries.append(
        dbcore.query.BooleanQuery('singleton', True, False)
    )

i.e., pass `False` for the `fast` parameter to indicate that this is not a
fixed-field query.