Fixed NPE in 'equal' when toleranceMode is missing.
Renamed TimeLimit -> TimeLimits

This has been named wrongly since the original JQTI. This is resolved
in this commit.

(Note that QTIWorks still does not enforce timeLimits anywhere.)
Added missing timeLimits.allowLateSubmission attr

This is now supported in JQTI+. (Note that timeLimits is not supported at
a "higher" level in QTIWorks at present.)