Add logo control

This adds a logo control, currently only used by the Bing Maps and MapQuest sources:

![logo-example](https://f.cloud.github.com/assets/6942/344844/3ede0882-9e0d-11e2-90b6-9ff07b1ac212.png)

This has the nice effect of keeping images out of the attribution control, which makes the attribution control less ugly (#336).

Note that the logo control's current default position (bottom left) will cause it to overlap with the scale line control. If you want both on the same map, then you'll need to tweak the CSS.