Add spm support

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