Add tests and bugfix for b'' messages appearing in python3.

I added unicode and non-unicode message testing for the output() method
on RTMBot. I'm concerned about why the encode('ascii', 'ignore') was
there in the first place, but not having it seems to do the right
thing...

rtmbot/core.py

@@ -101,7 +101,7 @@ class RtmBot(object):
                     if limiter:
                         time.sleep(.1)
                         limiter = False
-                    message = output[1].encode('ascii', 'ignore')
+                    message = output[1]
                     channel.send_message("{}".format(message))
                     limiter = True