added InvalidCategoryException.java;

added tests to categories: they should not be created with a name that is "", null and only be created with a color code where 0 <= code <= 0xFFFFFF

src/main/java/mi/hdm/exceptions/InvalidCategoryException.java

+package mi.hdm.exceptions;
+
+public class InvalidCategoryException extends RuntimeException{
+    public InvalidCategoryException(String message) {
+        super(message);
+    }
+}

src/test/java/mi/hdm/recipes/CategoryTest.java

 package mi.hdm.recipes;
 
+import mi.hdm.exceptions.InvalidCategoryException;
 import org.junit.Assert;
 import org.junit.Before;
 import org.junit.Test;
@@ -61,4 +62,24 @@ public class CategoryTest {
         //expect
         assertEquals(0, underTest.getAllCategories().size());
     }
+
+    @Test(expected = InvalidCategoryException.class)
+    public void canNotAddCategoryWithEmptyName() {
+        Category emptyName = new Category("", 0x1000);
+    }
+
+    @Test(expected = InvalidCategoryException.class)
+    public void canNotAddCategoryWithNullName() {
+        Category emptyName = new Category(null, 0x1000);
+    }
+
+    @Test(expected = InvalidCategoryException.class)
+    public void canNotAddCategoryWithNegativeCode() {
+        Category emptyName = new Category("foo", -100);
+    }
+
+    @Test(expected = InvalidCategoryException.class)
+    public void canNotAddCategoryWithCodeTooHigh() {
+        Category emptyName = new Category("foo", Integer.MAX_VALUE);
+    }
 }