add first class geojson 

so far I have:

`rTree.geoJSON(geojson);`

as a shortcut to add each of the geojson features to the rTree, though not GeometryCollection yet, i also added 

`rTree.bbox(sw,ne)` or `rTree.bbox([sw,ne])` as a short cut to `rTree.search({x:sw[0],y:sw[1],w:ne[0]-sw[0],h:ne[1]-sw[1]})`