TaskBook: code cleanup
TaskBook: code cleanup

Related task operations are grouped together.

While we are here, remove one unused overloaded form of resetData()

CS2103AUG2016-T11-C4/main,nightism/addressbook-level4,snehasp13/main,CS2103AUG2016-T11-C4/main,snehasp13/main,nightism/addressbook-level4