breaking changes for 0.2.x

[scottlamb]

Things I'm considering fixing:

• PasswordClient (and its friends plain and digest) return Result<_, String>. They should use a real std::error::Error impl instead.
• Each digest type should probably be an individual optional dependency, rather than having digest pull in both md5 and sha2 crates. In particular, I think it's rare for servers to use the SHA digest types, and folks might not want to have that dependency.
• Rename http_auth::digest::Algorithm::Sha512Trunc256 to Sha512_256 to match the change with sha2 version 0.10.0.
• ParamValue should probably be called ParamValueRef for consistency with ChallengeRef.
• http_auth::basic::BasicClient and http_auth::digest::DigestClient maybe should be http_auth::basic::Client and http_auth::digest::Client respectively to eliminate the "stuttering" in the type names.
• http_auth::PasswordClientBuilder could just take a with method based on a trait that adds stuff to the client, avoiding the need for somewhat arbitrarily chosen distinct method names for the str vs ChallengeRef vs http::HeaderValue vs http::GetAll cases

Other suggestions welcome.