JS updates and futureApi placeholders

-add non-breaking placeholders and functions for new api version
-store collection objects in their own dictionary instead of at top
level of Zotero.Collections object
-remove Item.contentRows which was left over from pre-JSON responses
-add ajaxRequest method with default settings
update bibcontent example with citations and server linkwrap
don't parse apiVersion since it is disappearing
use both group owner and ownerID for compatibility
PHP wrapDOI function for dx.doi links