Reverse Job->JobType relation

Make Job reference JobType instead of the other way around.
Should fix https://github.com/pyfarm/pyfarm-models/issues/25
Fix test_insert for TestTags and TestSoftware

Since Job now requires a valid jobtype to be created, these tests were
broken.

TestJob.test_getid() is not fixed by this and still broken
Remove get_job_id() and the respective test

After switching the job-jobtype relation around, you can no longer
create a job and insert it into the database entirely without data
anymore, so this function cannot possibly work
Merge pull request #30 from guidow/reverse_job_jobtype_rel

Reverse Job->JobType relation