Exam time limits Merged

Some operators would like to specify a time limit for writing an exam. This should be in addition to the current open and close date functionality.
For example: an exam is open for a week but has a one hour time limit set. The examinee can open the exam at any point during the week but once they have done so they get one hour to complete and submit it. If the questions are not complete within that hour they should be prevented from answering any further questions and should only have the option to submit the exam for marking.