Posting enhancements for Twitter, Statusnet, Pump.io and Wordpress

New features:
------------------
- Since Twitter and Statusnet can handle linefeeds they aren't suppressed anymore when posting.
- Posts to pump.io that contain youtube and vimeo videos now get a preview image.
- Youtube videos posted to wordpress are now embedded.

Bugfixes:
------------
- Due to an uncleared variable it could happen that a title from a previous item was also posted to a totally different item from a different person
- activity posts from statusnet now aren't mirrored anymore