Added Adjusted Miles, estimated miles driven per vehicle

Added two columns to the ZipCarLocations:
`adjusted_veh_count`: Down sampled the number of vehicles at each
location to match the vehicle count in 2013 Q4, which is the last
quarter for which Zip Car provided usage data.
`miles_per_veh`: For each location that still contains vehicles after
downsampling, the number of 2013 Q4 miles per vehicle.