node v0.10.x support

This change moves the c++ `decoder` part of tilelive-s3 to use node-gyp.
This also moves the minimum node version supported to 0.6.x. (0.6.13 as a minimum was used because that is what node-sqlite3 requires).

Needs tested (build + unit tests) before merging:
- [x] - OS X with homebrew based png
- [x] - Ubuntu with system png