Refactor Thing Description parsing in thing.ts

[benfrancis]

Every time I edit thing.ts I notice that there's a lot of logic duplicated between the constructor() and the updateFromDescription() methods.

This is a reminder to myself to try to untangle that and more cleanly extract out common methods for parsing each member of a Thing Description.

In the next W3C WoT Working Group charter period there are plans for a normative algorithm for parsing Thing Descriptions, so this class should eventually implement that algorithm.