[simple_button] Added a new option - shortcut - to said method

Will render a single character keyboard shortcut key on the button
[grading]: Displaying keyboard shortcuts for each button in the grading panel

Not being processed though, yet
Merge branch 'master' of heroku.com:drona
[grading]: Keyboard behaviour during grading defined

Comment box now does *not* get focus back automatically.
DB annotation changed
DB annotation changed
New query method to get QSelections for an AnswerSheet added
Cosmetic changes
[answer-sheets]: Storing a signature for each answer sheet

The signature is a a string with version numbers for each question
for that student - one character per question - starting from question #1
Versioning
- bug with number of vers per test (ans_sheet) fixed
- changed field name to match wsdl