Use Native Abstractions

[lovell]

There are compiler warnings with node v0.11 unstable.

../src/sharp.cc: In function ‘v8::Handle<v8::Value> Resize(const v8::Arguments&)’:
../src/sharp.cc:159:77: warning: ‘static v8::Persistent<S> v8::Persistent<T>::New(v8::Handle<T>) [with T = v8::Function]’ is deprecated (declared at /home/travis/.node-gyp/0.11.3/deps/v8/include/v8.h:5514) [-Wdeprecated-declarations]

Using https://github.com/rvagg/nan should hopefully reduce the impact of V8 changes, e.g. https://github.com/rvagg/node-postgres/commit/6cb167112ef65845dec818e7a888218fbe408483

[lovell]

Thank you to @pierreinglebert for implementing this - see PR #11