Edit homepage readme, update Ui.png, change output task filename

Add user guide to project

Add mass operation to mark and delete tasks

Users can now perform tasks on multiple items at the same time.

This helps to reduce the time taken to run similar tasks.

Add mass operation to mark and delete tasks

Users can now perform tasks on multiple items at the same time.

This helps to reduce the time taken to run similar tasks.

Improve code quality using the guidelines in the website

Use assert features in GuanGuan

Add abstract method to Task, fix GUI bye animation

Add GUI to chatbot using JavaFX

Add checkstyle to detect coding style violations

Add unit test for Deadline, Event, Parser, TaskList, Todo