MARCXML export and import mapping changes

* MARCXML export: change 852 and 555 mapping
* update tests and fix typo
* MARCXML import: fixup 300 mapping
* MARCXML import: fixup date mapping
* MARCXML import: subject mapping changes:
	* When there is more than 1 subfield with a given
	code, do the "right thing"
* Only make_term if !term_type.nil?
MARCXML export map changes

* Fix term mapping for subject headings
* Cleanup name property mappings
* Bring in agent source properly
* Make sure agent terms are added to subject fields
* Change note mappings
* Add container summary to extent
MARCXML import: description rules from 040$e
Merge pull request #579 from hudmol/pr-578

MARCXML import/export mapping updates
Add .yardoc and API.md to .gitignore
finalize import export mappings and tests
merge master and resolve conflicts
added instructions for building backend documentation
revise and refactor marc export tests
fix error raised when exported repo has no agent
Merge pull request #581 from hudmol/EAD-export-final-clone

Ead export final clone
Merge pull request #580 from anarchivist/gitignore-yard

Add .yardoc and API.md to .gitignore
stream EAD exports
load children objects at time of serialization
disambiguate fragments variable
EAD export: fix schemaLocation; add XLink namespace
EAD export: resource.ead_location should map to eadid[@url]
EAD export: place @langcode on language, not langmaterial
EAD export: Map linked agent role to @label
EAD export: wrap each note_definedlist item in <defitem>