fix exceptions inheritance

Traceback (most recent call last):
  File "./jenkins.py", line 12, in <module>
    print(jenkins[jobName].get_scm_type())
  File "build/bdist.freebsd-9.2-STABLE-amd64/egg/jenkinsapi/job.py", line 324, in get_scm_type
NameError: global name 'exceptions' is not defined
Merge pull request #172 from kworr/patch-1

fix exceptions inheritance