Parser class
Refactoring of command unit tests
Added equals function to check for equality between commands.
Currently, it will check if the commands had the same user input string
Changed Command class such that:
it now accepts a primary command for show.
it is now able to handle no primary argument for show.
It is now able to handle no primary argument for the other commands
Improved on parser in general.
Refactored method Parse(String) to parse(String)
Added some unit tests for Parser class
[Refactor] Changed CommandImpl.getIterator to CommandImpl.getAdditionalArguments
UI Class
[Refactor] Removed RETRIEVE from Command enum
showToUser in UI now throws an Exception
Input validation for CommandType
Input validation for Primary Operand and additional Arguments
Changed displayError to displayException
Added comments for Parser Methods
Implemented
feedbackForUpdate
feedbackForDelete
displayException
Part of feedbackForShow (lacking show type)
part of displayTodaysTask (can only show no task)
Separate class for main function and controller

Public main function in LDo class and Controller in a new class.
Added try-catch block in exeuteCommand()
Merge branch 'master' into controller/sharon

Conflicts:
	sg/codengineers/ldo/controller/LDo.java
Cancelled try-catch block
Run shows welcome message and today's task list

When L'Do is run, it will show a welcome message and today's task list.
Handled nullPointerException for _additionalArguments in Command
#34