Fix issue #38 Requiring png only exposes the Png() class

[scottmcginness]

Moves NODE_MODULE() into module.cpp so all classes get initialized

[ryan-williams]

ah, I was about to submit an identical PR. any chance of merging this into master, @pkrumins?

[mathiasbynens]

@pkrumins It looks like you don’t have time to maintain this project anymore. That’s fine, but please consider giving contributors commit access so they can merge these patches and release some updates. Thanks!