Buffer the SPT by 10 minutes and then filter post-IDW.

This checkin resolves artifacts around the edge of a travelspace by
buffering the maximum traveltime in the SPT by 10 minutes and then
filtering cells whose maximum traveltime is beyond the threshold.
Remove debugging URL.
Merge pull request #70 from joshmarcus/topic/buffer_for_idw

Buffer the SPT by 10 minutes and then filter post-IDW.