Hs item class

Created the Item class with some unit tests (more can/should be added) and Category as an Enum-class. Wrote some JavaDoc for Item class.

src/main/java/no/ntnu/idatt1002/demo/data/Category.java

+package no.ntnu.idatt1002.demo.data;
+
+public enum Category {
+
+    FOOD,
+    CLOTHES,
+    OTHER,
+
+}