Updated date validation for CREATE & UPDATE
Resolved conflict for Logic
Resolved conflict after View Refactor
Fixed bugs in assertion and LogicTest
Fixed tentative task output formatting

It was originally showing duplicate date ID for date ranges. Now it has
been fixed where one date range only gets one date ID.
Fix add task conflict logical error
Refactor checkConflict for better efficiency
Refactored and improved Logic on Conflicts
#75 check validity of task(deadline, floating, timed)
change Command to abstract class and add abstract method execute()
Remove a TODO in DatePair.isWithinPeriod()
Merge branch 'master' into pattern_refactor
Add XStream libraries
Switch to XStream XML encoder/decoder
Revert "Merge branch 'master' into pattern_refactor"

This reverts commit cdfd74984ef9d02851d8096d1341c4c9931f8d86, reversing
changes made to fddfc21a033e2e9799f95a1230fd02cb850f37a2.
change visibility of attributes in Command to protected
move some share methods from Logic(old) to Command
AddCommand extends Command
move some static methods from Logic(old) to Command
markCommand extends Command