use NAN for Node 0.8->0.11+ compatibility

Interested in this at all? I've bumped into problems with upstream dependents using unix-dgram and not having 0.11 compatibility.

Tests pass on all the versions I've tested:

```
Node@master  : PASS wrote output to /tmp/unix-dgram-dnt-master.out
Node@v0.11.9 : PASS wrote output to /tmp/unix-dgram-dnt-v0.11.9.out
Node@v0.11.8 : PASS wrote output to /tmp/unix-dgram-dnt-v0.11.8.out
Node@v0.11.7 : PASS wrote output to /tmp/unix-dgram-dnt-v0.11.7.out
Node@v0.11.6 : PASS wrote output to /tmp/unix-dgram-dnt-v0.11.6.out
Node@v0.11.5 : PASS wrote output to /tmp/unix-dgram-dnt-v0.11.5.out
Node@v0.11.4 : PASS wrote output to /tmp/unix-dgram-dnt-v0.11.4.out
Node@v0.10.22: PASS wrote output to /tmp/unix-dgram-dnt-v0.10.22.out
Node@v0.10.21: PASS wrote output to /tmp/unix-dgram-dnt-v0.10.21.out
Node@v0.10.20: PASS wrote output to /tmp/unix-dgram-dnt-v0.10.20.out
Node@v0.10.19: PASS wrote output to /tmp/unix-dgram-dnt-v0.10.19.out
Node@v0.10.18: PASS wrote output to /tmp/unix-dgram-dnt-v0.10.18.out
Node@v0.8.26 : PASS wrote output to /tmp/unix-dgram-dnt-v0.8.26.out
Node@v0.8.25 : PASS wrote output to /tmp/unix-dgram-dnt-v0.8.25.out
Node@v0.8.24 : PASS wrote output to /tmp/unix-dgram-dnt-v0.8.24.out
Node@v0.8.23 : PASS wrote output to /tmp/unix-dgram-dnt-v0.8.23.out
Node@v0.8.22 : PASS wrote output to /tmp/unix-dgram-dnt-v0.8.22.out
Took 10s to run 17 versions of Node
```