Add build script to automate a task over all supported akka versions

As requested, Erik.  Take a look and let me know if this helps.  Hopefully it can be used to automate the pain away :)

Running this will execute the command-line argument against the 3 specified akka versions.  It will also cross-compile.  If no argument is specified, it will default to package.  If you don't *always* need a cross compile, it may be better to specify the `+` on the command line.