Migrate to Native Abstractions (rvagg/nan)

Hi @dachev, with these changes I've managed to get your excellent module to [compile and pass all the tests](https://travis-ci.org/lovell/node-cld/builds/29246002) on Node.js versions 0.10, 0.11 and therefore (hopefully) the forthcoming 0.12.

In addition I silenced compiler warnings from the underlying CLD dependency and simplified the `NodeCld` namespace by removing the `Cld` subclass and associated header file.