[sparkle_appcast_publisher.rb] Fix changeset coalescing so it coalesces in the right direction.
[sparkle_appcast_publisher.rb] Git hides the full commit message in #getComment (which no other SCM plugin has)

Calling the method when it doesn't exist doesn't seem to kill jruby--it just
returns nil. Crazy.
Add message filter.

If you give a prefix then only the lines in your commit messages with that
prefix will show up in the appcast changelog (with the prefix stripped).
Add support for using markdown in appcast descriptions (built from commit messages).

This requires the pegdown-formatter plugin.
[README.md] Wordsmithing.
[README.md] Document the new changelog stuff.
[sparkle-appcast.pluginspec] Bump version up.

It's been fairly stable so far, so bump it all the way to 0.9