Added ability to switch basemaps

[bengolder]

This creates a new component, BasemapToggle.js When this toggle (a simple <select> element) is changed, it fires a ViewAction The active basemap is stored in MapStore. The Map needs to check if the basemap has changed when receiving new properties, similar to the way it handles new active layers. However, when the basemap changes, the entire map needs to be re-rendered, and any currently active layers need to be cleared and redrawn once the new basemap has a chance to load.

[bengolder]

@johnvf please give this a once-over when you have a moment.

[bengolder]

if merged, this would close #7

[johnvf]

Looks good to me