EAD export: include components' notes; refactoring

* make sure notes for components export! TODO: tests
* add several helper methods: `serialize_origination`,
`serialize_indexes`, `serialize_nondid_notes`,
`serialize_bibliographies`, `serialize_controlaccess`
* refactor serialize_did_notes to use `data` as its first
parameter, instead of an array of notes
* associate a few methods with `ArchivalObjectDescriptionHelpers`
instead of `ASpaceExport::model :ead`
EAD export: add tests for notes on components
Merge pull request #18 from anarchivist/ead-export-refactor

EAD export refactor