Add 'otherwise' wrapper method to route-segment

Hi @artch, 

_Your `angular-route-segment` not only added more features to `angular-route`, but also made the usage more elegant and simple!_

This pull request is to add 'otherwise' wrapper method to `route-segment`.
I know you want not to make unnecessary adjustment for `angular-route`. But this little change will make user codes more consistent, since you already adjusted the 'when' semantic.

The `route-segment` 'otherwise' can be used as below:
```js
$routeSegmentProvider.
    when('/', 's0').
    when('/1/a', 's1.a').
    when('/2', 's2').
    ...
    otherwise('/').
    ...
```

Can you help review and give your suggestions?

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_Thanks for your awesome job! :D_