Stub all the things

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.
Doing this check here instead of in `HasRefitHttpMethodAttribute` means that the method could return `true` for something that's doesn't actually have a Refit attribute (i.e. one with a different "Get" or "Post" attribute). I'm pretty sure it doesn't matter as long as we check somewhere.