Allow npm package to be resolved via node module resolution

Created by: jsdf

Bootstrap currently publishes an npm package but as there is no index.js or main field in the package.json, even if it is installed via npm it can't be resolved by require.resolve().

This would be useful when using Bootstrap in projects with a node-based build system (grunt, gulp, etc). For example, something like:

var path = require('path')

// using require.resolve
var bootstrapPath = path.dirname(require.resolve('bootstrap'))
// alternatively, just using require, as index.js exports __dirname
var bootstrapPath = require('bootstrap')

// compile some less with bootstrap in the import paths
require('less').render(someLessData, {paths: [bootstrapPath + '/less']} , function(err, data) {
...
})