Fix for instance-based views

When debugging instance-based views (such as those from [django-haystack](https://github.com/toastdriven/django-haystack)), a traceback is encountered:

```
Traceback:
File "/usr/lib/python2.7/dist-packages/django/core/handlers/base.py" in get_response
  105.                         response = middleware_method(request, callback, callback_args, callback_kwargs)
File "/home/joel/src/django-pdb/django_pdb/middleware.py" in process_view
  55.         filename = inspect.getsourcefile(view_func)
File "/usr/lib/python2.7/inspect.py" in getsourcefile
  444.     filename = getfile(object)
File "/usr/lib/python2.7/inspect.py" in getfile
  420.                     'function, traceback, frame, or code object'.format(object))

Exception Type: TypeError at /discover/
Exception Value: <search.views.ProductSearchView object at 0x7fce1c84bfd0> is not a module, class, method, function, traceback, frame, or code object
```

To fix this error, inspect the class instead of the instance.