UUID Generator

This generator uses the node-uuid package to generate a random uuid (v4).

```
var uuid = grand.uuid();
// => 97135ac5-ef50-4288-bdd6-ff8f19a8aa2a
```

It is the first external dependency for grand so I understand if it is problematic. There is always the option of porting a small fraction of code for generating a random uuid (v4).