Revert Airspace.isGameOver() so things build again
Rename main class in pom.xml
Added Assets.getDataFile so files work on all machines

- When using Maven and in the final JAR, the correct path is
  "data/uiskin.json"
- But when using Eclipse without maven, the correct path is
  "src/main/resources/data/uiskin.json"

This commit (and the function Assets.getDataFile) handle these differences.