Populate attribution control with layer attribution

Ensure that by default, attribution information is displayed when opting for attributionControl instead of infoControl.

So, in code, when creating a map like so:

    L.mapbox.map('map', 'examples.c7d2024a', { infoControl: false, attributionControl: true })

In the case of the map above, attribution is displayed like so:

![screen shot 2014-02-07 at 12 05 24 pm](https://f.cloud.github.com/assets/98233/2112128/0e7583f0-901a-11e3-8c16-afa78f88378d.png)

Right now, when creating map like in the example above, nothing is displayed in the attribution field.

This patch likely needs work as I'm not sure I'm passing the correct information in the call to `addAttribution()`.