add support for meta API endpoint
switch rate.Reset to be a Timstamp

depending on who people are using rate.Reset this may or may not be a
breaking change, since Timestamp supports all of the exported methods
from time.Time.
add support for new rate limit formats

mark RateLimit method as deprecated in favor of RateLimits which returns
both the core and search limits. (see deprecation notice in API docs:
https://developer.github.com/v3/rate_limit/#deprecation-notice)