Simple mod to enable better linux support

Node Serialport has some quirks under linux around how it discovers and provides back bluetooth serial ports which means that using the list ports and connect method a little flaky for linux use.

This mod just provides the option to be more explicit for linux users (or anyone really) by having the sphero library look for a argument passed in specifying the actual port to be connected to. This means on linux it's as simple as calling:

```
node example/cli.js /dev/rfcomm0
```

Or whatever the device is, and everything else works exactly as before.
No need to default this to a string. It's going to be undefined if it's not there.