Merge lp://staging/~hashar/python-jenkins/fast-job-existence-check into lp://staging/~python-jenkins-developers/python-jenkins/trunk

Branch contains a single patch that aims to speed up job existence test by lowering the amount of work that need to be done on the server side.

Previously job_exists() was calling get_job_info() which does an API call to 'job/%(name)s/api/json?depth=0'. That forces the Jenkins server to load all the build history which is potentially a long operation with a cold cache and definitely unneeded when we just want to verify whether a job exist.

Another step would be to update the other methods to rely on job_exists() instead of get_job_info(). Maybe with a new method like requireExistingJob() that will throw an exception whenever job_exists() returns None.