Override equals method in Task.java

Update README for the user guide

Override equals method in Task.java

Add ParserTest to test the commands

Handle duplicate tasks when adding the task to the list

Merge branch 'A-BetterGui'

Add the missing javadocs for methods

Add an icon for the app

Implement the archive feature with archive and unarchive command

A-Streams: Use Streams

Use stream to filter the keyword in findTasksByKeyword().

Use IntStream to map the index range from 0 to size of
the list in list().