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'd use `is instance(cls, basestring)` - http://stackoverflow.com/questions/1979004/what-is-the-difference-between-isinstanceaaa-basestring-and-isinstanceaaa