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.
Agreed that mongo implements this as radius, but I think "distance" is a better name. It also matches what is returned in the results a bit better ("dis"). So please change this param to "distance".