add hmac example
Merge pull request #116 from zhanming/master

add hmac example
Create ServeContent.md

终于算搞定这个文件了，困扰了几天了
Merge pull request #117 from qinglong1983/patch-20

Create ServeContent.md
Merge branch 'master' of https://github.com/astaxie/gopkg

合并最新版本