Cast job['AccountingGroup'] to a string

job['AccountingGroup'] is usually a string, but occasionally is a classad.ExprTree.
Cast it to a string so that split will work without error

Running jobview-update produced the following error:
> Traceback (most recent call last):
>   File "/usr/lib/python2.6/site-packages/htcondor_jobview/jobview_app.py", line 186, in application
>     return callback(environ, start_response)
>   File "/usr/lib/python2.6/site-packages/htcondor_jobview/jobview_app.py", line 42, in jobs
>     jobs, groups, schedd, user = summarize_jobs(cp)
>   File "/usr/lib/python2.6/site-packages/htcondor_jobview/jobs.py", line 71, in summarize_jobs
>     if 'AccountingGroup' in job:
> AttributeError: 'ExprTree' object has no attribute 'split'