Various archetype improvements

The executive summary is:

  * added a README with instructions; it seems better to have the instruction here, where they can be updated if the archetype changes; the Google Code page for the archetype could just link here
  * fixed issue with missing natives for desktop backend
  * moved assets to top-level, have other projects reference it from there
  * added "mvn integration-test -Pdesktop" for running things directly
  * added "mvn package -Pandroid -Psign" for signing and zipaligning APK
  * other little tweaks (like using tabs everywhere instead of half tabs, half spaces)