Core refactor

[mxstbr]

• Factor out all methods and make them testable
• Show what tests could look like for emitAssets util
• Export the main plugin file with module.exports = so users don't have to do var Atrium = require('atrium').default.
• Restructure the folders of the webpack plugin:

webpack-plugin
├── index.js
├── package.json
└── src
    ├── apply.js
    ├── assets
    ├── dynamic-resolve.js
    ├── loaders
    │   ├── examples-loader.js
    │   └── plugins-loader.js
    ├── registerDefaultPlugins.js
    ├── registerPlugins.js
    ├── utils
    │   ├── __test__
    │   └── emitAssets.js
    └── validateOptions.js