Job.set_owner and Job.get_owner added. They works with job owner property.
This property appears when "Job and Slave ownership" plugin installed.

I does not wrote unittests cause "Job and Slave ownership" does not provide api/jenkins.
Merge branch 'master' of github.com:dkochetkov/jenkinsapi

* 'master' of github.com:dkochetkov/jenkinsapi:
  Trailing spaces has been removed.
  Removed trailing whitespaces.
  Job.set_owner and Job.get_owner added. They works with job owner property. This property appears when "Job and Slave ownership" plugin installed.