In CommandGenerator:
1. added new method getActualRequest. this remove the firstword and get the rest of the
string.
2. refactored add, delete, edit, markdone (to ignore unimportant spaces).
    for example, if you type "    delete      2   ", it will still delete 2
    if you type"        add            fdfdff", it will still add fdfdff.
3. added new method containsUnnecessaryWords. this is implemented on display, clear,
undo, redo.
    for example, if you type "undo         XXXXX" . it will not undo because it contains
XXXXX.

In TaskManager:
1. modified the limiting condition for edit method

In junit tests,
revert everything back to Jireh's previous version

****** problems that need to be solved *********
****** every time when you exit then re-run the program, it will encounter index out of
bound error. It prompts something is wrong with the initializing process in the manager
class. ******