added auth header, refactored, added tests

[harlanbarnes]

• I added the auth header to the "jenkins list" call as opposed to JUST the build calls. Our jenkins installation is all behind an auth screen (not just the build calls). I did some testing that seemed to indicate that sending the authorization header for URLs that didn't need it had no real effect.
• Refactored ...
  • I moved the authorization handling into a single method called headers. I guessed that if later headers were needed they'd be easy to inject here.
  • The auth header is now optional, however, I doubt very many people have their Jenkins installation setup that way. 😄
  • I changed the caching of the job list to lazy load instead of explicitly calling the cache method. This was also important because there was a bug where if the user hadn't called "jenkins list" first and just tried to do a build, the build would fail.
  • Collapsed the jenkins_build_by_id into the jenkins_build method
• Fixed typo in the help output
• Changed the regex to cover both "jenkins b ID" and "jenkins build ID"
• Added tests to cover the "jenkins build" action

[daniely]

Sorry I somehow missed this. Looks great!