[Enhancement] Add the support of twitpic.com in Image Preview

[Contribucious]

Seems that Image Preview has the support for pic.twitter.com and (i.)imgur.com but not twitpic.com.
Therefore, clicking on a twitpic.com link open a tab in the browser rather than showing the image directly in Image Preview (and showing a thumbnail in the timeline in addition therefore).

Related to this (about the Instagram support):
https://bugs.launchpad.net/birdie/+bug/1190342

P.S.: Also, I see sometimes here and there some image links shown as "t.co" (can be in the timeline tab, search tab, etc.).
Need to check about that too so, but already reported as a response to an existing bug report, here: https://bugs.launchpad.net/birdie/+bug/1176097

TECHNICAL INFORMATION:
OS: Ubuntu 12.04.2 LTS (with Cinnamon)
Birdie version used: the r268 from the DAILY repository