Cleaning up sphero

This should put it to an [A](https://codeclimate.com/github/hybridgroup/artoo/Artoo::Drivers::Sphero). Some of the private methods should probably be pushed to the `Driver` class. I also wish there was a better way to handle messages.


Tests:

``` bash
$ rake
Run options: --seed 29185

# Running tests:

....................SSS..............S.............................
```