added spm support (accepts #31)

A nicer package manager: http://spmjs.io
Documentation: http://spmjs.io/documentation

http://spmjs.io/package/chroma-js

It is a package manager based on Sea.js which is a popular module
loader in China.

spmjs focus in browser side. We supply a complete lifecycle managment
of package by using spm.

requested by @sorrycc