twitter: improve optics and handling of retweets

Tweets were completely moved to the title and links are rendered as
html.
Retweets are handled the way intended by Twitter.
allow target attribute

Allowing the target attribute enables target=_blank directly in the
title.
Needed for Twitter links.
twitter: show tweeted media as thumbnails

Tweeted photos are directly integrated in the stream.
The alt attribute in the item.phtml had to be modified to reflect the
changes made to the title (having <a href> links in the alt attribute
is not allowed, so they must be removed).
make thumbnails bigger

Bigger thumbnails make Twitter photos more useful.

Note: this must be changed for mobile devices
fix thumbnail size on mobile devices

Having been enlarged for the Twitter media support the thumbnails are
now resized per css to match the screen width again.
fix link color in titles

Links in the title now have the same color as in the content.
Merge pull request #544 from Alcatr4zz/master

Add Twitter media and overhaul displaying tweets