Add tasks for all the new languages

Task/Stack/Swift/stack.swift

@@ -0,0 +1,26 @@
+struct Stack<T> {
+    var items = [T]()
+    var empty:Bool {
+        return items.count == 0
+    }
+
+    func peek() -> T {
+        return items[items.count - 1]
+    }
+
+    mutating func pop() -> T {
+        return items.removeLast()
+    }
+
+    mutating func push(obj:T) {
+        items.append(obj)
+    }
+}
+
+var stack = Stack<Int>()
+stack.push(1)
+stack.push(2)
+println(stack.pop())
+println(stack.peek())
+stack.pop()
+println(stack.empty)