Fix the download_all_resources

Fix the download_all_resources in XnatUtils. The download resource files per files generates an error if the resource has been upload as a zip before. This code fix it by actually downloading everything as a zip file and unzipping it.
Fixing download all resources

Fix the download_all_resources with the error on XNAT that randomly unzip a zip file when uploading it. IF only one file when download_all_resources, doesn't zip it, just download it and if it's a zip file, unzip it ELSE if more than one resource, download all of them as a zip and unzip it.
Update XnatUtils.py

Resource.label() to get the resource label name for the directory.
Merge pull request #12 from VUIIS/download_all_resource

Download all resource fixed.