OAuthSignatureGenerator that allows for any HttpMethod

This leaves `UrlSigner` within the code base to prevent any breaking changes, buy that now wraps and delegates to `OAuthSignatureGenerator` that takes a DTO (`OAuthSignatureInfo`) with a HttpMethod property that defaults to GET.

I've left alone the fact that the routing logic still only caters for GET and POST, which is something we'll have to deal with soon, but is probably best left as a separate job.

The only potential breaking change is that now UrlSigner is missing a default ctor, as it now takes an `ISignatureGenerator`. This is manually hooked up if you're using the concrete `FluentApi<T>`, but if not you'll have to rely on your DI framework to do it.