Added methods to encrypt / decrypt database IDs

Testpaper and student IDs will be encrypted to form the QR code

They will be decrypted during receive_scans and unresolved resolution
Split methods siblings? into 3 more precise queries

siblings_same_worksheet => same student, same worksheet
siblings_same_page => same_student, same worksheet, same page
siblings_same_page => same_student, same worksheet, same question
   (necessarily returns [] for standalone questions)
Quiz::layout? now returns in two different ways - depending on boolean argument

The first form is for sending as part of Savon request (default)
The second form is for internal usage - in particular when distributing scans
b/w examiners
For each examiner, we now have a running total of # of graded responses assigned and graded
New form QR codes being sent during BuildQuiz
Sticking with the same keys in hash returned by layout?
[big change]: New code for receiving and distributing scans

Expect bugs
Merge branch 'master' into dev