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
`File.expand_path(filename)` should be better so relative paths work cleanly.