Add a subscribePresence helper method.
Add a subscribePresence helper method.

peernohell/node-slack-client,slackapi/node-slack-sdk,slackhq/node-slack-client,slackapi/node-slack-sdk,slackapi/node-slack-sdk,slackhq/node-slack-sdk,peernohell/node-slack-client,slackapi/node-slack-sdk