Resample oneway paths to produce markers.

This uses a technique created by @mbostock in http://bl.ocks.org/mbostock/4965670
Need to confirm that this is faster. It is definitely cleaner.
Fix tests

Way lines are no longer the only paths on the map, so we need to be more
specific.
Corner-preserving resampling
Avoid unnecessary bind
Fine tune marker spacing
Merge pull request #918 from systemed/resample-oneway

Resample oneway paths to produce markers.