Merge pull request #4 from Suyooo/master

Bug Fixes
version 2.6
hasse cache
+ Started on a caching system. Right now it only caches API Data, not pictures
+ Added a setting to change the TTL of the cache data
+ Added a refresh button (which doesn't override the cache, though)
* Translated Picture Upload dialog to be more consistent with the rest of the UI
* Meal Detail fragment always shows the latest picture instead of a random one
* Sigfood API: SigfoodApi is now serializable, and has a field for the date
  of the call