SourceCodeClassifier marks unused open declarations

![image](https://cloud.githubusercontent.com/assets/873919/3530402/235c77fe-07a4-11e4-92f4-95802ae5e057.png)

I will add tests but it basically works. 
It may not have any influence here. The idea of having `qualifyOpenDeclarations` to return `Async<_>` is to mark it as a long-running operation (it's obvious from the function signature). If we use `Async.RunSynchronously` liberally, it's fine. But in general, when we start using it, we lose the ability to reason about whether things are executed on GUI or background threads.