DspaceCollection methods for single items

[PoorBillyPilgrim]

Define workflow, methods, and data model for single item from given collection. This will be information added to README eventually.

// item param is folder name for item in collection
collection.getItem(item) // returns contents of item folder
collection.getItemMetadata(item) // returns dublin_core.xml contents as a JS object
collection.getItemJson(item) // returns dublin_core.xml contents as a JSON string
collection.getItemImage(item) // returns file name of image for item
collection.getDcvalues({item: item, descriptions: descriptions}) // returns metadata as jsonld for Manifest