add a buffer to fitSelectedRoutes so pins don't get cut off

[burritojustice]

When a center and zoom level are not specified for a map, it results in a very snug fit and the pins end up getting cut off. (Might be LRM's fitSelectedRoutes?)

Would be nice if we could add padding to whatever fitbounding is going on.

[hanbyul-here]

Making this as an official api of lrm-mapzen will take some consideration and time. Meanwhile, I added the example of how to do this here : https://github.com/mapzen/lrm-mapzen/blob/gh-pages/fit-routes-with-padding.html