Added sbt-assembly for desktop

While it is easy to create/deploy application for Android thanks to sbt-android
and proguard, there was no easy way to build application for PC. sbt-assembly
approach seems to be one that "just works" without any specific configuration,
so one can run:

    $ sbt assembly

and get nice jar bundled with everything needed to run application, including
scala libraries, libgdx and shared libraries.