Add File To Question

* add: carrierwave
* add: Attachment model
* add: AddQuestionIdToAttachment
* add: q has_many attachments
* add: accepts_nested_attributes_for
* add: = f.fields_for :attachments
* add: @question.attachments.build
* add: li = a.file.file.filename
* add: attachments_attributes: [:file]
* add: link_to a.file.file.filename, a.file.url
* add: delegate :identifier, to: :file
Merge branch 'nested_forms' of https://github.com/tropikan4/qna into nested_forms