Added test runner.

% bin/run-tests

✓ loads .sbt_completion.sh
✓ fails to start sbt on empty project
✓ starts sbt on empty project if -sbt-create was given
✓ downloads sbt 0.7.x
✓ downloads sbt 0.10.x
✓ downloads sbt 0.11.x (< 0.11.3) from org.scala-tools.sbt:sbt-launch
✓ downloads sbt 0.11.x (>= 0.11.3) from org-scala-sbt:sbt-launch
...