Handle exec() call with options (env, pty = true, etc).

I'm currently using the `ssh2` module and I call conn.exec with an options object before the callback. As `ssh2-multiplexer` is, right now, I get this:

```
TypeError: Property 'callback' of object #<Object> is not a function
    at /path/to/node_modules/ssh2-multiplexer/connectionqueuer.js:38:20
    at /path/to/node_modules/ssh2/lib/Channel.js:361:5
    ...
```

This happens because your module assumes the original call is simply `exec(cmd, callback)`. This PR handles the options object and detects whether it was passed or not.