Adding config property for varying cache headers

Adding config property for varying cache headers.

Verified headers with 7-day cache (`604800 == 7 * 24 * 60 * 60`):

```
Access-Control-Allow-Origin:*
Cache-Control:max-age=604800
Content-Length:28777
Content-Type:image/jpeg
Expires:Fri, 22 Aug 2014 17:16:26 GMT
```
Comment should probably be more general, about converting days to seconds, since we may not always set this to 1 year.