Mapbox attribution control

This commit adds a new attribution control in reference to #485 

I stuck with an _info_ icon as this felt visually stronger as an icon as is more diverse than a &copy. I've also placed this control in the bottom left corner (as opposed to the bottom right) as it created some awkwardness when a legend is just above it.

Here's a little animation of it. The control appends any attribution content passed in to `L.mapbox.attributionControl.addAttribution()` and adds an "Improve this Map" link by default. There's also a supported removeAttribution method. 

![attribution](https://f.cloud.github.com/assets/61150/1579449/7e0800e8-519b-11e3-86f9-984279ffb096.gif)