Update jqgrid.py method get_colmodels

Update jqgrid.py method get_colmodels:
when the queryset has fields of aggregation (Count, etc. ....), these fields do not have a corresponding field object in the queryset (FieldDoesNotExist), in which case, we must provide a  basic colmodel.
Merge pull request #2 from mbanaouas/master

Update jqgrid.py method get_colmodels