Offline mode for data

[choefele]

Offline mode would be a great addition to the app because AppFog availability is often flaky and the user might not have any network connectivity.

The idea is to use YapDatabase to persist data.

Tasks

• Store stolpersteine in DB by ID
• Unique downloaded stolpersteine
• Extend StolpersteinSynchronizationController to send delegate methods when data changes
• Read from DB at app start
• Optimization: only download missing data from server (server support needed)
• Use full text search for querying stolpersteine instead of server request

New APIs must have unit tests.

[choefele]

This is being worked on on the offline-sync branch