Several modifications have been done to the code in MainActivity.java
and Single_Player.java

Changes:
Implimented OnClickListener for both MainActivity and Single_Player
class. 

Created OnClickListeners for each button in both classes. Also created
onClick() method in each class, to handle all clicks from
OnCreateListener. 

This onClick() method does a select case comparision to determine which
view was clicked in order to call the appropiate methods.