Remove transitive interfaces (implicitly inherited ones)

Before:
![uml-old](https://f.cloud.github.com/assets/776829/1820564/3acf0174-70f3-11e3-8d50-1b29ebffaada.png)

After:
![uml-new](https://f.cloud.github.com/assets/776829/1820563/3aa83bca-70f3-11e3-8173-f1e64d584031.png)

Fixes #10.
Not sure how this relates to #17.