Added synchronous download of device test results

Ideally, I guess you would want this functionality to be subsumed into appthwack-java but it looks like AppThwackRun#downloadResults() isn't implemented yet: I have tried to keep all the download code in one single method (getTestResults), so that it can be refactored out at some future date.  At this stage, I haven't changed any of the UI for the plugin, nor made the results download optional, but that would be easy to do.  Device test results are stored in the workspace in a folder named "device-results-<run id>", with the individual devices as sub-folders as per the downloaded zip file.