Add an observable starred client

Hot on the heels of putting together a `StarredClient` for this project, i want to continue on with the starring part of the Activities API by adding an observable client for starring.

## todo

* [ ] Implement the `IObservableStarredClient` interface
* [ ] Add tests for the client

Have y'all had enough? Or are you thirsty for more?

![Macaulay Culkin - Home Alone](https://f.cloud.github.com/assets/238079/1538142/71354b6a-4cdb-11e3-8f6f-ea0e612c6443.gif)