Rate limiting fixed to work as described in the hipchat's API v2 docs

Hi,

I hope you find this pull request useful. It works for my use case where I have to make hundreds of requests to the API. The readme.md says this is a python package for HipChat's v2 JSON API and that's why I made it to conform to the specs regarding [Rate Limiting](https://www.hipchat.com/docs/apiv2/rate_limiting). 

Kind Regards