Minor enhancements to rate limiting information

If rate limiting information hasn't been retrieved already, it will be checked (via the /rate_limit endpoint, so the request won't count against the limit) 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 opted for a new attribute rather than a third value in the `rate_limiting` tuple).

RateLimiting unit test updated accordingly. I am pretty confident that I did the right thing in recording the replay data, but please have a look at this to be sure.

Additional new attributes may be required once the search limit API is finalized.