Add common authorization directives

[gcotelli]

After merging #20 this can be done sending the generic setAuthorizationTo: to the header builder, but it could be good if we have some specific methods for common cases (like we do with the Bearer token case).
See

• https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Authorization
• http://www.iana.org/assignments/http-authschemes/http-authschemes.xhtml