Bump version number to 1.2.4.0

And update the changelog.
There should be no incompatible API changes or incompatible
behaviour changes. There are a number of compatible API extensions
and behaviour fixes. In particular this gives us a limited amount of
forward compatabilty with Cabal-1.4, ie there are things that work
in 1.4 that would break with 1.2.3.0 but should work with 1.2.4.0.