Rough draft share feature

In prep for sending out to users, a quick draft of the share feature. 

Appears when you click share, and automatically selects the link for easy copying. The box disappears when your mouse moves back over the map (same thing we do for feedback widget). This is slightly annoying, but is probably a decent temporary interaction.

Longterm, we should think more carefully how panels appearing / attach to the main sidebar box. 

![screen shot 2014-05-15 at 1 40 26 pm](https://cloud.githubusercontent.com/assets/1435836/2990531/091c702c-dc72-11e3-81e2-2636ffaca279.png)