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
Also this is going to simply store the response body regardless of the response. So checking if the response code is a success (200) before saving the body should be good.