Use more NAN for Node 0.11 compat

tbh I can't compile this so I'm not even sure if it's all correct but this uses NAN a bit more to give you the 0.11 compatibility you'll need. The method signature has completely changed in 0.11 so you have to use `NAN_METHOD` to make it work with both. Also `Persistent` handles are different and you can't just directly assign to them in 0.11, hence pulling it up to a static and using `NanAssignPersistent`. Also `NanSymbol` is a simple shortcut for symbols.