fix: turn allGroceries into property, not function

cfr. https://github.com/xtofl/growsery/issues/2

The allGroceries() function was 'pure', but not idempotent
like required by angular.

This caused successive $digest cycles to keep detecting
changes.

https://github.com/angular/angular.js/issues/6054#issuecomment-36748726
shows that making it into a regular property, and installing
a watch on it's input properties, should do the trick.