feat: Enable logging with the singleton pattern

Resolves #4

Author: tthuem

ProfCalculator/src/de/uulm/sp/swt/profcalculator/Logger.java

@@ -0,0 +1,23 @@
+package de.uulm.sp.swt.profcalculator;
+
+public class Logger {
+	
+	private final static boolean LOGGING = true; 
+	
+	private static Logger logger = new Logger();
+	
+	public Logger() {
+		log("Logging enabled:");
+	}
+
+	public static Logger getLogger() {
+		return logger;
+	}
+
+	public void log(String message) {
+		if (LOGGING) {
+			System.out.println(message);
+		}
+	}
+
+}

ProfCalculator/src/de/uulm/sp/swt/profcalculator/ProfCalculator.java

@@ -54,9 +54,11 @@ public void handle(ActionEvent event) {
 			int newValue = Integer.parseInt(inputField.getText());
 			if (event.getSource() == additionButton) {
 				expression = new Addition(expression, new Value(newValue));
+				Logger.getLogger().log("+ " + newValue);
 			}
 			else if (event.getSource() == multiplicationButton) {
 				expression = new Multiplication(expression, new Value(newValue));
+				Logger.getLogger().log("* " + newValue);
 			}
 			expression = new NecessaryBrackets(expression);
 			updateGUI();