Fix changelog formatting

[brainwane]

Twine's changelog, as represented on Read The Docs and when I build the docs locally, only displays version number 1.4.0 (2014-12-12) and two associated GitHub issues. This is inaccurate since changelog.rst has 70+ items and runs up to version 1.9.1 (2017-05-27).

This is happening because changelog.rst is meant to be formatted so the Sphinx extension releases can parse it, and releases misparses what's in there right now.

I'm working on this in my changelog-fix branch and, mostly by fixing indentation and newlines, I've gotten a lot more information to display, but several items that have already shipped in releases are at the top of the page and marked as waiting for future releases. I think this is because we basically always ship bugfixes in what releases thinks of as feature releases, operating similarly to "unstable prehistory" mode, and so I need to be inelegant and explicitly mark these items explicitly as each applying to/included in the next release.

Work in progress.

[brainwane]

(We could also switch to using Sphinx's built-in changes builder.)

[sigmavirus24]

I'm not sure the changes builder will work that well. There are lots of places where we can't mention something changing such that it would appear in the changelog.

[brainwane]

OK. I'll fix up this branch to work with releases, then.

[brainwane]

Fixed in #305.