Added a RateLimit class to encapsulate pulling rate limit information
from the headers. This is now exposed by ApiInfo as well as the
RateLimitExceeededException. That way these two classes are not grabbing
the same information in different ways which we were doing before.
Right now, our exception messages come from the API response in most
cases. But if for any reason there isn't one, we supply a default. I
realized these messages might make it to people's logs so it might be
nice to spend time later making them even more useful.
