use NAN for Node 0.8->0.11+ compatibility

Not sure if you're interested but [NAN](https://github.com/rvagg/nan) provides an easy means to maintaining compatibility from 0.8 onwards, the V8 changes in particular in 0.11 are a big deal and make it a nightmare to keep a module compatible.

Just ran [DNT](https://github.com/rvagg/dnt) across it on Linux and I get passes:

```
Node@master  : PASS wrote output to /tmp/nan-dnt-master.out
Node@v0.11.9 : PASS wrote output to /tmp/nan-dnt-v0.11.9.out
Node@v0.11.8 : PASS wrote output to /tmp/nan-dnt-v0.11.8.out
Node@v0.11.7 : PASS wrote output to /tmp/nan-dnt-v0.11.7.out
Node@v0.11.6 : PASS wrote output to /tmp/nan-dnt-v0.11.6.out
Node@v0.11.5 : PASS wrote output to /tmp/nan-dnt-v0.11.5.out
Node@v0.11.4 : PASS wrote output to /tmp/nan-dnt-v0.11.4.out
Node@v0.10.22: PASS wrote output to /tmp/nan-dnt-v0.10.22.out
Node@v0.10.21: PASS wrote output to /tmp/nan-dnt-v0.10.21.out
Node@v0.10.20: PASS wrote output to /tmp/nan-dnt-v0.10.20.out
Node@v0.10.19: PASS wrote output to /tmp/nan-dnt-v0.10.19.out
Node@v0.10.18: PASS wrote output to /tmp/nan-dnt-v0.10.18.out
Node@v0.8.26 : PASS wrote output to /tmp/nan-dnt-v0.8.26.out
Node@v0.8.24 : PASS wrote output to /tmp/nan-dnt-v0.8.24.out
Node@v0.8.25 : PASS wrote output to /tmp/nan-dnt-v0.8.25.out
Node@v0.8.23 : PASS wrote output to /tmp/nan-dnt-v0.8.23.out
Node@v0.8.22 : PASS wrote output to /tmp/nan-dnt-v0.8.22.out
Took 15s to run 17 versions of Node
```