http header中cache设置
http://blog.csdn.net/wangjj_016/article/details/5714226
http://www.360doc.com/content/08/0501/14/41237_1227846.shtml
http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.3.4
http://www.mnot.net/cache_docs/
杨建：网站加速--Cache为王篇 http://www.itlearner.com/article/4421
Caching in HTTP http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html
Header Field Definitions http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html
age http://labs.apache.org/webarch/http/draft-fielding-http/p6-cache.html#header.age
Age Calculations http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html
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