Add fetch! method, change name of delete method

Added a method, `fetch!` for Jobs.

`fetch!` tries to fetch a Job and all of it's asset paths.  If it fails to fetch asset paths (Because Sauce has returned a 400, as the job is incomplete), `fetch` will now return a job without asset paths for screenshots.  `fetch!` will throw a `Sauce::JobNotComplete` exception.

Renamed `Assets.delete_asset` to `Assets.delete`.