v0.1.3 — automatic updates

Automatic updates are here. From this version on, Thunderbird checks this
repository for new releases and updates the add-on on its own — no more manual
reinstalls. (Thunderbird does not require add-on signing, so this works without
any signing step.)

One last manual step

This is the version that introduces the auto-update mechanism, so it has to
be installed by hand once:

1. Download tb-thread-summarizer-0.1.3.xpi below.
2. Thunderbird → Add-ons Manager → gear → Install Add-on From File….

From 0.1.4 onward, updates install automatically.

Full details in the
CHANGELOG.