Make rate limit headers optional

[Tom-Fawcett]

It would be a useful feature to make the setting of the following response headers optional

X-RateLimit-Limit: 200
X-RateLimit-Remaining: 0
X-RateLimit-Reset: 1394506274
X-RateLimit-Bucket: authorized-users

In some scenarios you may not wish to expose these to the API consumer.