Write a (now failing) test for issue #36 that tests that the submission time of a project is recorded in the grade book.
Use java.time.LocalDateTime to represent assignment submission dates.
Address issue #36 by noting the time the project was submitted in the grade book.
Added (currently failing) tests that validates that an assignment's submission time is persisted in the Student's XML.
Broke up a big method in XmlDumper to make it easier to work with.
Refactored code in XmlStudentParse to make it easier to work with.
Persist the submission times in the Student's XML file.
Refactor parts of XmlDumper to make it easier to work with.
Add a "due date" to an assignment in the grade book to address more of issue #36.
Fully address issue #36 by marking a project that was submitted after its due date as late.
Refactor AssignmentPanel to make it easier to work with.
Added support for the grade book UI to display and set the due date of an assignment.
Refactored GradePanel to make it easier to work with.
Use a java.time formatter to format a java.time LocalDateTime.  Old DateFormat don't work with new times.  I don't know why I thought that would work.
Make a Grade as dirty when a new submission time is added.
Display the submission times of an assignment on the "Grade" panel of the grade book UI.
Merge pull request #96 from DavidWhitlock/issue-36-mark-submissions-as-late

Address issue #36 by adding a submission date/time to grades and a "due date" to assignments. When a project is submitted after its due date, it is marked as "late". All of that information is available in the grade book UI. Along the way, I cleaned up a lot of the Student and GradeBook XML parsing/dumping code.