Slow down when connection is slow

[joto]

This is often used to present OSM at some kind of events. Often the internet connection isn't that great, so it takes some time to load the images. Before they get a chance to be loaded the location changes, so the sat images are often not visible that much. Maybe some function to slow everything down? Maybe just by pressing + and -?

[tmcw]

There's a customizable runSpeed variable - this could be controlled with a slider.

[iandees]

See also #31, which I filed because panning and loading the Bing tiles is what takes up most of the CPU time.