Move prebuild.xml so that we can build the module simply by cloning the git repo as $OPENSIM_BASE/addon-modules/Whisper

Otherwise this is just a constant pita when you want to make development changes.
This commit also adds more data to the README about building the OpenSimulator module
This does build with OpenSimulator 0.8 and current dev, though don't know for sure if the end to end stuff works.