Made it possible to not log actor crashes

This functionality is intended to be used together with `Celluloid.exception_handler`, and relates to questions in #459. The background is that sometimes you want to completely take control over how exceptions are logged. (in our specific use case, we want the formatting to be different, to be more closely aligned to other non-Celluloid-exceptions are being logged)

If you feel this should be done in some other way, just let me know. I'm using a monkey-patched version of Celluloid for the time being, where these modifications are included, and it works quite well for me.