Handle EADDRINUSE

no ref
- Show a human readable message on EADDRINUSE

---

Prevents the all too common stacktrace below and shows a nice message instead.

```
events.js:72
        throw er; // Unhandled 'error' event
              ^
Error: listen EADDRINUSE
    at errnoException (net.js:901:11)
    at Server._listen2 (net.js:1039:14)
    at listen (net.js:1061:10)
    at net.js:1143:9
    at dns.js:72:18
    at process._tickDomainCallback (node.js:459:13)
    at process._tickFromSpinner (node.js:390:15)
npm ERR! weird error 8
npm WARN This failure might be due to the use of legacy binary "node"
npm WARN For further explanations, please read
/usr/share/doc/nodejs/README.Debian
```
I think this should probably be `process.exit(0)` since we're handling this error.  Otherwise, if Ghost is started via npm, npm will throw a fit if it gets a non-zero exit status.

Thoughts?