Degen ring testcase (+fix)

- Refactors tiled segment creation to just convert polygon rings to tiled coordinates (refs #53) (still drops degenerates)
- Fixes localMin/localMax checks which previously did not identify start/end min maxes properly in certain cases. See https://github.com/mapbox/tile-cover/blob/degenring/test/unit.js#L40-L226 for new correct behavior.