Improved subscription of channel based on hangups

[averissimo]

Motivated to work better on bad connections as I was getting connection reset when trying to subscribe

Can someone test this to make sure all functionality is kept?

see:

• https://github.com/tdryer/hangups/tree/master/hangups/client.py#L406
• https://github.com/tdryer/hangups/tree/master/hangups/channel.py#L232