Muzei Widget

Add a simple widget which allows the user to open the activity and skip the current artwork displayed.

A user of my Tumblr for Muzei extension sent the request so I thought I would take a crack at it.
http://jimmithy.tumblr.com/post/86502807543/hello-there-first-of-all-i-would-like-to

What do you think?

![widget](https://cloud.githubusercontent.com/assets/1126762/3056683/6c35491c-e1cd-11e3-8b06-3a414456d60b.png)