Debug text for ServiceDescriptor

Fixes https://github.com/aspnet/DependencyInjection/issues/113

How it looks after this change:

![untitled](https://cloud.githubusercontent.com/assets/5921642/5206464/83f41f0c-7556-11e4-811b-ab5ad8e462f3.png)

Please review @suhasj 
Is the pipe `|` typical in debugger displays? It looks like an "or" to me.