Convert subject attribute extraction to use System.ComponentModel

I'm planning to add subject attributes dynamically from machine.fakes. This way no `SubjectAttribute` would have to be added in explicitly on specs that derive from `WithSubject<>`.

For this to work the attribute has to be added and extracted through System.ComponentModel instead of normal reflection.

I could not test this in the ReSharper runner, but it works all right in the console runner. As far as I could see the ReSharper runner uses the same logic to extract the subject?

I have not tested this for performance, but can do if you think this could be an issue.