signing with purpose via :for option

Work on #8 

There're many `options` everywhere :sweat_smile: But it seems hard to avoid them..

But I have some questions, so I hope to get some of your help :smiley:  @jeremy .I'll comment on the code
ok, I'll just use "default" purpose. And, I think we should add doc to locator.rb about purpose after this's done, can I do this?