Adds documented makeIssues.xsl stylesheet.

This is a cleaned-up, documented version of the stylesheet
I've been using to convert a title-level MODS record with
constituent issues into a tree of stub METS and MODS records.
Adds keyDate attribute to dateIssued point='start' elements.

Every constituent needs a keyDate, so its id can be computed.
Merge branch 'master' of https://github.com/pulibrary/BlueMountain

Conflicts:
	metadata/periodicals/bmtnaaz/bmtnaaz.mods.xml

Natasha changed a date; I accepted her change.