Cache results within timeframe

[devinmatte]

Right now we call Walkscore directly for each call. Because we only allow a subset of stations to be searched, we can easily cache the results to avoid extra hits to walkscore within a range of probably 24hrs (walkscore isn't changing on an hour by hour basis)

[bhnzhang]

Hey @devinmatte, wanted to see if this is still a relevant issue? I think I could take a crack at it