Make caching more robust

[mikaelsvensson]

The current app cache basically relies on mirroring the remote database in the app's own database. This is obviously not efficient since the in-app database could potentially contain a lot of unused data.

Articles which may be of interest:

• https://android-arsenal.com/tag/14
• http://developer.android.com/guide/topics/data/data-storage.html
• http://eclipsesource.com/blogs/2012/07/31/loading-caching-and-displaying-images-in-android-part-1/
• http://www.technotalkative.com/android-load-images-from-web-and-caching/

At the same time, it might be a good idea to investigate if it is possible to reduce the number of SQL queries when loading long search results.