[welcome.js] Wait for user to come online before welcome message

[samuelcouch]

The problem I've noticed is that creating a Slack account automatically throws you in to #general, but that doesn't necessarily mean that you're online and in the chat.

Right now we catch the robot.enter event (a user joins a channel) and push new users into the queue to welcome them at the next interval -- what would be best is push them into a queue and wait until the next time they come online and welcome them at that time (this could still be batched).

Requires finding a way to detect online status.