use binding.gyp instead of bindings.gyp, also fix requiring

binding.gyp are enforced by https://github.com/TooTallNate/node-gyp/commit/c071ab3a4c80fc3bc57780eec6505ca829803c70
Merge pull request #11 from anru/buildtype

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