get current build artifact

Noted https://github.com/arangamani/jenkins_api_client/issues/61 and created a custom method to find and download the current build.

I'm sure it could use some cleanup I'd love to see what that is.

An example of how I'm using this can be seen here https://gist.github.com/joelneubert/19f607887a7246925a17
It would be nice to raise with an error if the response code is not what we expect (`200`).