Help messages done
Fixed Bug is Sort by End Date
Interpreter supports mark undone.
Change and modify the search part

(1) "Search By Keyword" can only search by words.
-e.g. Task name: Go to Jurong East
can be search by"go to", "go jurong"....
CANNOT be seached "o t", "to J"
(2) can be search by any items
linked GUI to help message. GUI can print strings