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