Added calculator assignment.

calculator/calculator.py

+#!/usr/bin/env python
+
+class Stack(list):
+    def push(self, element):
+        self.append(element)
+
+    def __str__(self):
+        return str(self[-1])
+
+
+class Calculator(Stack):
+    def add(self):
+        self.push(self.pop() + self.pop())
+
+    def sub(self):
+        temp = self.pop()
+        self.push(self.pop() - temp)
+
+    def mul(self):
+        self.push(self.pop() * self.pop())
+
+    def div(self):
+        temp = self.pop()
+        self.push(self.pop() / temp)