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.
add .travis.yml
paranoiac string quoting
unset system's `JAVA_HOME` to force tests to use stubs
add build status in README
Merge branch 'unittest' into develop