Allow SlackClient rtm_connect exceptions to be raised

[mtingers]

The SlackClient method rtm_connect does not provide a reason for
failed connections and only returns True or False. This change
makes it so:
- Exceptions can be raised in the client's rtm_connect method
- A connection can be established on instantiation

For example, this exception can now be raised so the problem is not
masked:
urllib2.HTTPError: HTTP Error 429: Too Many Requests

The slack client can now be instantiated in this manner:
sc = SlackClient(SLACK_TOKEN, connect=True)

Exceptions can also be raised by invoking rtm_connect as such:
sc.rtm_connect(debug=True)

[pkinlen]

It really would be helpful to have the debug argument:
sc.rtm_connect(debug=True)

[palfrey]

Any chance of moving this forward?

[Mixser]

Please, change name of "debug" argument, because it's make confuse, that i mustn't use this flag in production. 'raise_exception' better than 'debug'

[KellyR-STCU]

Please lets move this forward. I currently cannot connect and I have no idea why because it just returns False.

[Roach]

Closing this in favor of refactoring a bunch of error propagation logic in here