- introduce pairsByKeys() (from http://www.lua.org/pil/19.3.html but adding current index and total length as iterator return values)
- use pairsByKeys() everywhere, where contacts are iterated over (for now only when displaying in gui.lua) so the order stays the same although the structure is a hashmap
- correctly update selected position when adding/removing contacts to TO/CC/BCC fields
- remove print statements in storage.lua
- restructured storage.lua so reading/writing json is not duplicated
- when a player joins and has no contacts file yet, automatically add all players he wrote to
