Make nodes of untraversed files gray

When running objc_dep on a subset of a larger project, it can be useful to know which classes are dependencies but weren't traversed - say if they were in another folder. This makes it easier to tell if a class really has no dependencies.

![t](https://f.cloud.github.com/assets/1820237/1693877/0e427e56-5e97-11e3-9887-aa32b21c8533.png)