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.
I was tempted to put also check that `a.ArgumentList.Arguments[0].Expression` is a string literal, but that would stop someone from using a constant. I think it makes the interface less readable if the expression has a constant so maybe we just make the call that constants are dumb and don't allow them. 

If we want to check for a constant, do we also need to allow for constants that reference other constants? It could get messy.