Download older launchers if the version is declared in `build.properties`
Set `sbt.global.base` expectedly
add tests for sbt-extras based on sstephenson/bats

[Bats](https://github.com/sstepehnson/bats) is a testing tool for bash
scripts. It has been used in famous bash projects like Ruby's rbenv.
unset system's `JAVA_HOME` to force tests to use stubs
paranoiac string quoting
add .travis.yml
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
...
Merge pull request #68 from paulp/add-bats-test

Add test runner.