Changelog 0.3

Updates to the changelog before 0.3 release.  I tried to exhaustively include references to every user-visible change that is actually a difference between 0.3 and the 0.2.5.  This is probably not 100% perfect, but close enough.

The amount of time this took (even with the help of a bit of automation) suggests I have not applied enough pressure on people to include changelog entries for their changes (though some people have been very good about it).