npm support (CommonJS and ES6)

[discapes]

Basically how it works is the main code is still in lib/chessboard.js. lib/ also has 3 wrapper files for different environments (CommonJS, ES6 and browser). The build script minifies lib/chessboard.js and wraps it and outputs the files to dist/. I published the package to npm, and instructed on the usage in README.md and the website download page.