form name->identifier + refactoring

I changed API of the gem to be more friendly for usage in real usage scenarios

example:
```
client_attrs = {age: 18, first_name: "John", last_name: "Smith"} # grabbed from DB
Clerkapp.print("some-questionaire-id", client_attrs)
```

I also adapted code to recent API changes in printouts: attribute name changed to form_identifier