Stub anything that looks like a Refit interface

This should resolve #65. `InterfaceStubGenerator` now looks for any interface in a file with `using Refit;` that has at least one method with a `[Get|Head|Post|Put|Delete(.*)]` attribute on it.

I have a couple of concerns about the approach, but I'll comment in-line so it makes more sense.
I'm not sure what you mean? I think that's what I'm doing here - my point was just that I'm not doing it in the method that looks for Refit attributes. 

I could take the rolled up list all usings here and pass it in as a parameter, then it's not going to be traversing the whole tree each time the method is called. I'm conscious of the fact that this runs as part of the build, so we need to make it as fast as possible. Maybe it's still micro-optimising though?