Refactor build routine and switch to airbnb style (#1703)

- Relocate webpack stuff to assets/build
- Switch all js to airbnb code style
- Update webpack to v2.0 (beta)
- Update all other dependencies
- Assets manifest uses relative path as key [1]
- Code should be compatible with Node 4.5.0
- All dependencies have been updated

[1]: Previously the assets manifest would use the basename of a file path as
its key when looking up a cachebusted file. This was to be consistent with
Sage 8. This change makes it so that the relative path is used instead.

To clarify, review the following example.

Before:
```
{
  "main.js": "scripts/main_5f4bfc9a9f82291c6dea.js",
  "main.css": "styles/main_5f4bfc9a9f82291c6dea.css",
  "customizer.js": "scripts/customizer_5f4bfc9a9f82291c6dea.js"
}
```

After:
```
{
  "scripts/main.js":"scripts/main_5f4bfc9a9f82291c6dea.js",
  "styles/main.css":"styles/main_5f4bfc9a9f82291c6dea.css",
  "scripts/customizer.js":"scripts/customizer_5f4bfc9a9f82291c6dea.js"
}
```

src/lib/Sage/Asset.php

@@ -21,8 +21,7 @@ class Asset
     public function __construct($file, ManifestInterface $manifest = null)
     {
         $this->manifest = $manifest;
-        $this->asset = basename($file);
-        $this->dir = dirname($file) != '.' ? dirname($file) : '';
+        $this->asset = $file;
     }
 
     public function __toString()
@@ -33,6 +32,6 @@ class Asset
     public function getUri()
     {
         $file = ($this->manifest ? $this->manifest->get($this->asset) : $this->asset);
-        return get_template_directory_uri() . self::$dist . '/' . $this->dir . '/' . $file;
+        return get_template_directory_uri() . self::$dist . "/$file";
     }
 }