Add option to pass access_token in Authorization header instead of query parameter

[sbellity]

This proposal allows to pass the access_token as Authorization header to FB api instead of query parameter.
Does not change the current behavior which is still the default, but adds a UseAuthorizationHeader on the session to enable this feature.

example use:

func newFacebookSession(ctx context.Context, accessToken string) facebook.Session {
        session := facebook.Session{}.WithContext(ctx)
        session.SetAccessToken(accessToken)
	session.UseAuthorizationHeader()
        return session
}

[huandu]

LGTM. Thanks for your contribution!