OAuthSignatureGenerator that allows for any HttpMethod

*right, trying again.....*

This leaves the original `UrlSigner` within the code base to prevent any breaking changes, but that now wraps and delegates to `OAuthSignatureGenerator`, and 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.

Relates to Issue #100