Improve handling of single job imports

Hi, noticed a problem with job-import-plugin when importing single jobs.

The url http://somejenkins.com/jenkins/job/jobname/api/xml?tree=jobs[name,url,description] doesn't lead anywhere for me in the current version of jenkins. Reason: can't get an xml tree for jobs from a single job.

Fix below first checks if url submitted is for a single job then gets the appropriate xml document