Fix timing code in HttpRequestThrottler


If you're using the World Weather Online free API, which permits at most 3 requests per second, the HttpRequestThrottler class might make more than 3 requests per second (the timing code isn't perfect.) I don't believe the C++ REST SDK currently reports server events, but perhaps there's a way to more accurately throttle the requests such that you don't exceed the 3 requests/second constraint.

(You may get an email from World Weather Online stating that you've exceeded the throttle limit, but the app will continue to function.)