implement jobs logged to database

Hi,

This is a pull request to ask if you are interested in merging something like what I'm using. Changes:

- JobRun model to save data about running/run jobs.
- Job model to create jobs and exec them later. Admin action still TODO. Will act as a repository of known jobs
- Admin configuration to see the JobRuns
- Beanstalsk_loggedworker command: copy of beanstalk_workder command with a new launch_job function that encapsulates JobRun creation and updates.

Please let me know if you would like something like this merged in django-beanstalkd.