Renaming method

From #build_header_doi_form to #build_assign_a_doi_form. I believe
the new method name to be more descriptive and parallel to the other
method that is being crafted (i.e. #build_request_a_doi_form)
Renaming action/views

From #assign to #assign_a_doi. Prefering the more verbose description
instead of the somewhat more opaque version.
Renaming class

From the more opaque HeaderDoiRequestFrom to RequestADoiForm
Renaming class

Renaming HeaderDoi to AssignADoiForm.
Merge pull request #1 from jeremyf/refactoring-names

Refactoring names