Add design of data structures and storage

[GlenDC]

The stored data should:

• be able to stored and used easily locally;
• should be possible to query on it (e.g. SQL) so we can easily get stats on it;
• should be possible to sync cheaply with server (e.g. simply copying an entire db file won't work for several reasons, most importantly that it is very expensive to do and it doesn't allow a clean bi-directional sync for correct or real-time purposes);