added a twig extension class for assets + reorganized assets structure + added command line for stylus in README

lib/renderer/assets.php

@@ -0,0 +1,65 @@
+<?php
+namespace MailPoet\Renderer;
+
+class Assets extends \Twig_Extension {
+
+  public $assets_url;
+
+  public function __construct($assets_url) {
+    $this->assets_url = $assets_url;
+  }
+
+  public function getName() {
+    return 'assets';
+  }
+
+  public function getGlobals() {
+    return array(
+      'assets_url' => $this->assets_url
+    );
+  }
+
+  public function getFunctions() {
+    return array(
+      new \Twig_SimpleFunction(
+        'stylesheet',
+        array($this, 'generate_stylesheet'),
+        array('is_safe' => array('all'))
+      ),
+      new \Twig_SimpleFunction(
+        'javascript',
+        array($this, 'generate_javascript'),
+        array('is_safe' => array('all'))
+      )
+    );
+  }
+
+  public function generate_stylesheet() {
+    $stylesheets = func_get_args();
+    $output = array();
+
+    foreach($stylesheets as $stylesheet) {
+      $output[] = '<link
+        rel="stylesheet"
+        type="text/css"
+        href="'.$this->assets_url.'/css/'.$stylesheet.'"
+      >';
+    }
+
+    return join("\n", $output);
+  }
+
+  public function generate_javascript() {
+    $scripts = func_get_args();
+    $output = array();
+
+    foreach($scripts as $script) {
+      $output[] = '<script
+        type="text/javascript"
+        src="'.$this->assets_url.'/js/'.$script.'"
+      ></script>';
+    }
+
+    return join("\n", $output);
+  }
+}