add optional "radius" http param

For example, the url can end with  ../bikes?lat=41.3979520&lng=2.18004200&radius=1000

That searches within 1000 meters. 

if you don't add the radius param, it uses the default of 100 meters.
Please use isNaN() as on line 30 below, to help catch the case where user passes &distance=eggplant.