Add spm support

A nice package manager for browser: http://spmjs.io
Documentation: http://spmjs.io/documentation

http://spmjs.io/package/sortablejs

---

It is a browser side package manager which is a popular module loader in China. It supply a complete lifecycle managment of package by using [spm](https://github.com/spmjs/spm/tree/master).

> If you need ownership of sortablejs in spmjs, I can add it for your account after signing in http://spmjs.io.