Modified StartScreen, Hangman and WordList classes.

Added functionality for start button, start screen, animal button and
validation of not selecting category.
Merge branch 'master' of https://github.com/singhalsurbhi07/202-Hangman

Conflicts:
	Hangman.class
	Hangman.ctxt
	Hangman.java
	StartScreen.ctxt
	WordList.class
	WordList.java
	project.greenfoot
StartScreen, Button impl