npm postinstall script

I often forget to run `npm install`, `bower install` and `grunt` before run `npm start`, so I'd like to put `postinstall` script.

This is similar stuff with `pure-site`.
https://github.com/yui/pure-site/blob/aa9167fcdb73abcf857a2642e775cb1fbbfd85a3/package.json#L67