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 it might be a good idea to break the body of the `for` loop out into its own function. Otherwise the `continue` part of the logic is easy to overlook / a little hard to reason about (had me confused for a bit).