Got rid of `process.exit()` in `/bin/mrt.js`

There was following lines:

```
  // TODO Annoying, why do we have to do this?
  // Shouldn't the app terminate naturally?
  process.exit();
```

I've investigated this issue a bit and found out that Node closes application automatically iff there are no events that can occur in the future (or they don't have any callbacks). DDP client creates a socket on connection and sets `message` handler for it. This handler can, obviously, fire anytime, that's why Node won't close Meteorite while there is at least one established connection .

I've added `Atmosphere.disconnect()` method, which closes current connection (if any) and removes the `Atmosphere._client` object. This branch successfully passes all 126 tests on my Debian 7 machine.

Waiting for your comments.