Sortable.js breaks browser tab shortcuts

Shortcuts like click+cmd, click+ctrl in browsers that allow opening links in the background is broken.
ctrlKey and metaKey (metaKey is the cmd key on the mac) usually open links in new tabs in the background, Chrome does it like this.
shiftKey in Chrome opens a link in a new window.
Merge pull request #600 from stipsan/patch-1

Sortable.js breaks browser tab shortcuts