Show twitter user timeline in new tab

# Summary

I am using this blueprint to consider the benefit of changing the twitter user profile links to cause the specified user's timeline to be loaded into a new tab within gwibber, rather than opening their profile page in an external browser as it currently does. This concept could then be extended to other supporting protocols, e.g. identi.ca, to reduce external browser usage.

# Implementation

Have put a draft implementation of this into the attached branch. To test, simply checkout the branch and ./run it. Then click on the avatar image for one of your twitter friends to see their timeline in a new tab, or click any of the inline @username links for the same behaviour.

# Known problems

If the user has 'protected their updates' this currently results in an empty tab being displayed. Twitter API returns the http status header '401 Unauthorized' and also basic json {"request":"\/statuses\/user_timeline\/username.json","error":"Not authorized"}. Need to decide how best to handle this.

# One More Thing...

Feedback welcome.