Add X-RateLimit-Reset to response headers

As per https://developer.github.com/v3/#rate-limiting the
X-RateLimit-Reset header is included in every response and is the time
in UTC epoch seconds when the current ratelimit window resets.
Regen API with X-RateLimit-Reset response header
Add /rate_limit API

Retrieves the current rate limit status for both core and search.

See https://developer.github.com/v3/rate_limit/
Regen API to include /rate_limit
Add test for misc.rateLimit

Generated, then modified