Add tests

I added tests for sbt-extras with using [sstephenson/bats](https://github.com/sstephenson/bats), which is a testing framework for bash projects. Plus, I fixed some potential problems in sbt-extras to pass tests.

1. Set `sbt.global.base` for pre-0.13
1. Skip adding empty string as sbt argument when `$log_level == "Info"`
1. Download older releases of `sbt-launch.jar` if specified
1. Quote bash variables to avoid unexpected expansion