Map can be dragged off the screen

[aslagle]

From Zach:

On the Map view, if you scroll N/E/S/W you enter white space. You should either not be allowed to scroll past the map (Google maps does this for N/S scrolling), or scrolling should lead you back to the other side of the world (google maps strategy for E/W scrolling).

[jbeezley]

We've had this for a while: OpenGeoscience/geojs#57

The east-west wrapping is more complicated, but it won't take me long to prevent panning the map off the page.