Added the private method that appends the future meetings list to the
DOM document, completing the building of the doc. Also, refactored the
public methods loadFile and writeFile to load/saveData to distinguish
from the private methods that will do the actual file interactions.
Completed implementation of saveData -- it calls separate private
methods for clarity, first to create a blank DOM document, then to build
the document tree from the lists of contacts and meetings in memory,
then to convert the document to an XML string to save on disk. 
Written with a lot of help from online tutorials and hints linked in the
code!
loadData partially implemented: now calls a private method to extract
the contact data from the XML DOM tree. As well as using this data to
populate the list of known contacts, it also fills a map from IDs
to contacts -- this is to aid in the loading of meetings, as the
participating contacts are stored in the XML by ID.