Start on the 0.2.5.5-alpha changelog.

I've copied the entries from changes/, labeled the ones that also
appeared in 0.2.4.22, sorted them lightly with a python script
(added to maint), and combined sections with the same name.

I didn't combine sections without a description (e.g. "Minor
bugfixes:"), since we'll probably add a description to those.
Reformat 0255 changelog. Tweak formatter script.