BaseLayerPicker widget

NOTE: This is a pull request into the (now mis-named) sceneModeWidget branch.

Don't look at the code yet (because I still have cleanup to do) but I would love some feedback on the widget layout and usability.  I don't expect a code review until this is in the sceneModeWidget branch and ready for master.

This pull adds ImageryWidget, this widget provides a set of dropdowns with icons and labels and developers can supply imagery providers to grow/shrink the choices dynamically.

CC @emackey I'm sure my css is a mess and could use some cleanup, by let me know what you think overall or if there any glaring problems with my approach.  I'm still tweaking and playing with things.

P.S. Knockout templates are awesome.

Here's a screenshot of it expanded:
![image](https://f.cloud.github.com/assets/1495196/294466/e02a78de-93ee-11e2-84d1-d2a470a33774.png)