[notready] 1.x: vt query async

- [x] add async for queryMany
- [x] add async tests for queryMany
- [ ] changelog
- [x] corresponding master PR (https://github.com/mapnik/node-mapnik/pull/310)
- [ ] update API wiki (https://github.com/mapnik/node-mapnik/wiki/API)

cc @springmeyer @rsudekum