Issue #2: Adding new tab support for mac os x. 

Check for meta key (cmd) during click event.

References:
https://developer.mozilla.org/en-US/docs/DOM/event.metaKey
http://stackoverflow.com/questions/5604538/how-to-check-cmdclick
http://www.w3schools.com/jsref/tryit.asp?filename=tryjsref_onmousedown2