Separate the function that generates payloads from the one that POSTs them.

Hi, I'd like to use google-measurement-protocol with my Tornado application. Since Tornado is non-blocking, I can't use Requests. This pull request allows me to generate Google Measurement Protocol requests with your library, but I actually POST the request to Google using Tornado's non-blocking HTTP client.

Example usage of the new `payloads()` function is [here](https://github.com/ajdavis/motor-blog/blob/052935cf02d9f7caa0e2243c51898a9589032723/motor_blog/web/lytics.py#L41).