revised benchmarking system
make benchmark run against local libmapnik
benchmark: fix formatting
travis: run benchmarks
benchmarks: correct shebang
geojson parser : only close path if we have at least 3 vertices
geojson generator : catch empty geomeries
geojson parser : support 'null' as valid coordinates property (empty geometry)
Revert "geojson parser : support 'null' as valid coordinates property (empty geometry)"

Per GeoJSON spec :

```
A GeoJSON geometry object of any type other than "GeometryCollection" must have a member with the name "coordinates". The value of the coordinates member is always an array. The structure for the elements in this array is determined by the type of geometry.
```

This reverts commit f9ed2ce4e2d623c14df247db0025a549be29d75c.
cleanup commented out code
geojson generator : catch empty geometries early

```json
{"type":"Feature","id":1,"geometry":null,"properties":{}}
```
geojson parser : add phoenix v2 not_empty implementation