Add Node.js version of "identity" example

This is a Node.js implementation of the "identity" example in the "Identities and Addresses" section of chapter 3. It also includes a basic version of `s_dump` extracted into a separate module.

Sample run:

```
$ node identity.js
----------------------------------------
[005] 00800041a7
[000]
[026] ROUTER uses generated UUID
----------------------------------------
[005] PEER2
[000]
[033] ROUTER uses REQ's socket identity
```