Don't snap to stops using a decimal degree threshold

[mattwigway]

In the shapefile strategy, we use a decimal degree threshold to snap to stops. This means that everywhere except the equator we have buffers around the stops that are tall and skinny (because degrees of latitude are [roughly] constant length, about 111km, whereas degrees of longitude vary from 111km at the equator to 0km at the poles). The math to convert to meters isn't hard if you assume a spherical earth.