Update analysis job objects

This adds some functionality to job objects that is needed for the commands selection page. Exists now properly checks the data used for the job, and delete now works for jobs.
@squirrelo, see [here](https://github.com/greengenes/Greengenes/blob/master/greengenes/db.py#L82). Not perfect, but the end effect is the code is [leaner](https://github.com/greengenes/Greengenes/blob/master/greengenes/db.py#L205). It is common on large apps to do this, and actually go further by taking them out of the same source and sometimes fully centralizing.