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
@jeremy Adding a default purpose will break this test for default serializer can't parse and array.So the test is ugly.Can we modify our default serializer to support parsing for array?