Extend API data fetchers to transparently cache results as JSON files

This is a pre-condition for the design and implementation of #100 (closed).

Additionally it builds an important basis for the testability of Engine.

changed milestone to %1.0-alpha2 — Precise Pheasant

added P1 + 1 deleted label

assigned to @david

marked this issue as related to #100 (closed)

added To Do label

added 1 deleted label

added Doing label and removed To Do label

mentioned in issue #100 (closed)

closed

removed Doing label