Add tasks for all the new languages

Task/Mutual-recursion/Swift/mutual-recursion.swift

@@ -0,0 +1,16 @@
+func F(n: Int) -> Int {
+  return n == 0 ? 1 : n - M(F(n-1))
+}
+
+func M(n: Int) -> Int {
+  return n == 0 ? 0 : n - F(M(n-1))
+}
+
+for i in 0..20 {
+  print("\(F(i)) ")
+}
+println()
+for i in 0..20 {
+  print("\(M(i)) ")
+}
+println()