Upgrade to nan 2 for Node.js 4.0.0 compatibility

[NameFILIP]

"With Node.js 4.0.0 Nan took the chance to do some refactoring and also bumped its version from 1.x to 2.x, introducing breaking changes." (source)

Useful info for migration:
nodejs/nan#402
nodejs/nan#371 (comment)
https://nodesource.com/blog/cpp-addons-for-nodejs-v4

[NameFILIP]

Just noticed, there is already a PR for this: #42

[spoon1312]

Any update on this ? :)