New instance feature, authentication support, and multiple minor new features

• Support instantiation. You can create a SAN instance to share configurations (base URL, headers, authentications, etc.) across requests. Instance can also passed between different structs via Swift environment objects feature.
• Make authentication easier. You can use a username and password combination to construct a Basic auth or pass in a token for a Bearer one — no manual Authorization header is needed.
• Shortcut methods update. You can emit a request by SAN.PUT() as well as SAN.DELETE() instead of using SAN.request("PUT"). This feature is also available in instance.
• Readme update.