Support request gzip content.

Current CDN almost support auto gzip text base content mirrors.
e.g.: http://registry.qiniudn.com/npm
It can save ~650kb through gzip 700kb json content to 38kb.

![image](https://f.cloud.github.com/assets/156269/2301465/901bddae-a146-11e3-9aae-2f8a0be03d80.png)