#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
UpdateCommand extends Command
DeleteCommand extends Command
ConfirmCommand extends Command
nothing
HelpCommand extends Command
move showhelp()
viewCommand extends Command
SearchCommand extends Command