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
```ruby
raise PurposeMismatch, "Expected a Global ID signed for #{options[:for].inspect}, got one signed for #{parsed_purpose.inspect}"
```