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.
Is the ```jobid``` not an attribute of the ```Job``` itself?  I thought that it inherits ```id_``` from ```QiitaObject```, so it could just be an instance method that takes no arguments (besides ```self```), right?  Or am I missing something?