Add deprecation notice to Slack and SlackRTM backends

[sijis]

Adding deprecation notices to these backends and suggesting the SlackV3 one.

In the logs, the message looks like

00:03:28 INFO     errbot                    webhooks:  Flag to bind /github to notification
00:03:28 INFO     errbot                    webhooks:  Flag to bind /test to test
00:03:28 WARNING  errbot.backends.slack     This backend is deprecated and will be removed in a future release. No future updates, bug fixes or enhancements will be included. We strongly advise migrating to SlackV3, which is available at https://github.com/errbotio/err-backend-slackv3
00:03:28 INFO     errbot.backends.slack     Verifying authentication token
00:03:28 INFO     errbot.backends.slack     Connecting to Slack real-time-messaging API
00:03:29 INFO     errbot.backends.slack     Connected
```