Add sendPrivate method

Closes #75

- Add new `IrcResponse` class derived from `Response`
- Add `sendPrivate` to `IrcResponse`
- Add `sendPrivate` to `IrcAdapter`

Tested by altering the help.coffee script to have the following code:

```coffee
    if msg.sendPrivate
      msg.sendPrivate emit
    else
      msg.send emit
```

/cc @rjhornsby