Support for presence subscriptions over RTM

[devth]

Slack changed the API. Now we only get presence_change events if subscribed as documented here:
https://api.slack.com/docs/presence-and-status#subscriptions

For instance, to subscribe to presence_change events for users U123456 and W123456, write the following JSON blob into an established websocket:

{
    "type": "presence_sub",
    "ids": [
        "U123456",
        "W123456"
    ]
}