Add new Sleuthkit OsAccount objects to the case

[lfcnassif]

While working on #232, I got some aborting exceptions caused by objects without parents. Debugging, they are new OsAccount objects populated into TSK DB. For #232 I'll just ignore those new objects, but we should handle them properly adding that useful info to the case as new items, with their specific properties (login name, full name, creation time, SSID, account type, etc...)