Add support for HTTP.auth chainable

Allows for shortcutting HTTP Basic Auth and OAuth 2.0 Bearer Auth requests.

Example:

    HTTP
      .auth(:basic, user: "admin", pass: "admin")
      .get("http://website.com/path/to/restricted/area")

    HTTP
      .auth(:bearer, "[ACCESS TOKEN]")
      .get("https://website.com/path/to/restricted/area")