upgrade nan to 0.8.x

I got below trace when I `npm install hashring` at my mba, and `node -v` prints `v0.11.10`:
```
node_modules/hashring/node_modules/nan/nan.h:518:3: error: unknown type name 'uv_work_t'
```
I guess this is the 0.3.x nan is not available for now, then may I ask u merge this PR and `npm publish` to let  we can use this module at v0.11.x?

Thank you :P