Added Caching Headers

[manishparanjape]

Fix for #52

[tjwiebell]

@manishparanjape - Thank you for this submission, it really helped track down some odd symptoms we saw with Fastly in the cloud. These missing headers were not only causing content to not be edge cached, but also broke gzip compression/encoding in the process.

While I don't think we want to open up header injection, we do want to enforce this best practice of cache control. We're struggling to think of a use case why this would need to be extensible, but wanted to check with you. Can you think of a reason we shouldn't just append a year long cache-control header if one is not already present?