V4

- [ ] rename to v4
- [x] $type values mapbox/mapbox-gl-style-spec#69
- [x] halo width units mapbox/mapbox-gl-style-spec#66
- [ ] halo blur units mapbox/mapbox-gl-style-spec#66
- [ ] promote glyphs to a top level element mapbox/mapbox-gl-style-spec#77