Added a logout() method.

Prevents stale sessions being left on the server when not using cookies.

Based on code by user gofetch (https://github.com/gofetch/whatapi/commit/2190eba70f305dffdd1693162852d7f1336fad37 and https://github.com/gofetch/whatapi/commit/adba6ac539e08204fe2815768fb5adb231029d8b)