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. 
You could use Seq.choose and pattern matching on LongIdentWithDots here. It's not nice to have ident.Lid.Length traversing the whole list in the filtering step.