bbp user online status

Simply adds an «Online» or «Offline» under the author information in bbpress topics and replies.

Parts of code adapted from Robin Wilson’s bbp profile information, Raul Illana’s Awebsome! Online Registered Users Widget, and from onetrickpony at stackoverflow.

TODO

• Make inactive time configurable
• Include images in addition to text (configurable)
• Make location configurable
• Make text configurable