use NAN for Node 0.8->0.11+ compatibility

I know a similar PR has been submitted previously but I'm having a go now because we're getting late in the 0.11 cycle and it'd be nice to start playing with serialport on 0.11.

V8 instability isn't going to improve post-0.12 and most of the instability for native addons comes from V8's major changes and have almost nothing to do with Node. This is where NAN comes in, we're maintaining an abstraction between V8 & Node and native add-ons where the breakages happen and continue to adapt as more breakages happen and will do so beyond 0.12.