Update to api.php

- Hope it will be useful. I made these changes while trying to use this api in a standalone fashion. It looked like FastlyAPI::post was stuck  with only using POST, and not PURGE, but PURGE seems to be working fine. 
Instead of calling post here, I wonder if we should just have this method perform the PURGE request itself? Basically make it so that the `post` method below is true to its name and doesn't actually have any machinery for doing PURGE requests.