Add ability to filter out frames for creating a more useful stack traces

Currently the stack traces reported by a call to bugsnag.Notify() or bugsnag.New() result in stack traces that include a bunch of frames from the bugsnag package itself. This patch skips bugsnag frames and optionally allows the user to specify further frames to skip if they so desire. A simple example is a logging package that notifies in certain cases not wanting to include all the log package frames when reporting an error.