MDL-39337 html_writer: Added img() function

img function has been added to the html_writer.
use it by calling the function as shown:
html_writer::img('foo.com','Foo',array('title'=>'Foo2'));
This will result in the following string:
<img src="foo.com" alt="Foo" title="Foo2"/>

lib/outputcomponents.php

@@ -1101,6 +1101,22 @@ class html_writer {
         return $output;
     }
 
+    /**
+     * Generates a simple image tag with attributes.
+     *
+     * @param string $src The source of image
+     * @param string $alt The alternate text for image
+     * @param array $attributes The tag attributes (array('height' => $max_height, 'class' => 'class1') etc.)
+     * @return string HTML fragment
+     */
+    public static function img($src, $alt, array $attributes = null) {
+        $attributes = (array)$attributes;
+        $attributes['src'] = $src;
+        $attributes['alt'] = $alt;
+
+        return self::empty_tag('img', $attributes);
+    }
+
     /**
      * Generates random html element id.
      *