Minor enhancements to rate limiting information

If rate limiting information hasn't been retrieved already, it will be checked (against the `/rate_limit` endpoint) rather than just assuming 5000 requests remaining of 5000.

Add an additional attribute `rate_limiting_resettime` to reflect github's new `X-RateLimit-Reset` header. This integer value is a unix timestamp. (In order to avoid breaking anyone's code, I added a new attribute rather than adding a third value to the `rate_limiting` tuple.)

RateLimiting unit test updated accordingly. I'm pretty sure I did the right thing in re-recording the replay data, but please double-check this.