Added support to view chats

- Added InboxPage to view exsitent conversations
- Added Chat page, takes id of the chat partner as argument
- Added related Client functions to retreive messages from the server
- Added Converters
- TimestampToRelativeTime converts a timestamp with seconds since
1970-1-1 to a relative time
- Two other converters that are intended for use in the Chat page
- Added temporary Inbox button to main page
- Added sample data