Use logging instead of traceback (#309)

Instead of just outputting to stderr the rtm connect
failure it is more appropriate to use the python logging
system and output a appropriate message (and pass in
exc_info=True so that the traceback gets logged as well).

Author: harlowja

slackclient/client.py

@@ -2,11 +2,13 @@
 # mostly a proxy object to abstract how some of this works
 
 import json
-import traceback
+import logging
 
 from .server import Server
 from .exceptions import ParseResponseError
 
+LOG = logging.getLogger(__name__)
+
 
 class SlackClient(object):
     '''
@@ -52,7 +54,7 @@ def rtm_connect(self, with_team_state=True, **kwargs):
             self.server.rtm_connect(use_rtm_start=with_team_state, **kwargs)
             return self.server.connected
         except Exception:
-            traceback.print_exc()
+            LOG.warn("Failed RTM connect", exc_info=True)
             return False
 
     def api_call(self, method, timeout=None, **kwargs):