exception_level_filters: support string for filtered class name

https://github.com/rollbar/pyrollbar/issues/37

Added ability to specify exception level filters using either string or class type.
Added unit test for exception_level_filters



I think this is breaking things in python 3 (note the test failures). See https://groups.google.com/forum/#!topic/django-users/hCqNuEQKwso