Belt tensioner

After seeing the Ultimaker Belt Tensioner by psync (http://www.thingiverse.com/thing:10034), I decided to model my own that didn't need a screw. But of course there's also Roboteer's Paperclip belt tensioner (http://www.thingiverse.com/thing:4535)

Note: It's been mentioned in the comments that there isn't enough space between the new style Ultimaker slider blocks and the belts. Which causes the tensioner to hit the blocks.