use binding.gyp instead of bindings.gyp, also fix lib exporting if buildtype=debug

Requiring "compress_buffer_bindings" in index.js fails if node built in debug mode (node-gyp also built in buildtype=debug). This patch fixes this problem by official way.

Name "binding.gyp" are enforced by https://github.com/TooTallNate/node-gyp/commit/c071ab3a4c80fc3bc57780eec6505ca829803c70