State current value of Access-Control-Expose-Headers
Add X-RateLimit-Limit to existing examples

For each example that includes X-RateLimit-Limit &
X-RateLimit-Remaining, update the example to also include
X-RateLimit-Reset.
Define X-RateLimit-* headers
Show X-RateLimit-Reset in header

  - Update the /rate_limit example output to show the X-RateLimit-Reset
    header
  - Update #headers to allow callers to override the X-RateLimit-Limit
    and X-RateLimit-Remaining headers
Add blog post for X-RateLimit-Reset header
Show conversion from epoch time to JavaScript Date
Merge X-RateLimit-Reset docs