Add Travis CI configuration file, build status icon, and add setup script.

Using before_script, setup OpenSSH and create keys, files for unit test.

I have confirmed the build success at my repository.
https://travis-ci.org/zaneli/scala-ssh/builds/24152529
[![Build Status](https://travis-ci.org/zaneli/scala-ssh.png?branch=travis-ci)](https://travis-ci.org/zaneli/scala-ssh)