Documentation

tutorial
classes for Github objects
- how to get instances of them
- properties
- methods (and arguments)
api and how it is wrapped
rationale:
- lazyness for objects returned by API, not for objects requested by user
- naming: get_xxx() to avoid clashes with attribute xxx (User.followers for example), and to explicit api calls. One get_ <=> one api call. No get_ <=> no api call, most often, and one from time to time to complete an object.
- lazy completion, but no caching
- explicit edit instead of writeable attributes
- data model (cf Design.md)

Functional improvements