修补 README 文件中关于 secret_key_base 配置方法的错漏。
添加 variants 之前，产品需要有 option types

* 如果产品仅有一个 Master Variant 则在编辑产品页面显示 sku 字段
Update theme css and layout.

* Fix off screen nav
* Fix .nav-xs text hide on Firefox
* Fix .nav a focus with grey background
编辑产品的时候可以关联 Option Types

[Issue #9]

* Fix: order by string bug. string will cause ambiguous column name bug.
Merge pull request #42 from wjp2013/develop

Develop