Add a build task to create a jarjar dist artefact.

build.xml - Add a build-jarjar and dist-jarjar targets that generate a
jarjar'ed version of the dist JAR. This uses a customized build of
jarjar containing a fix for [1] (required because ServiceLoader metadata
needs to be rewritten in some dependent JARs).

build.py - Add a dist-jarjar task that calls the relevant ant target

[1] http://code.google.com/p/jarjar/issues/detail?id=30