Allow to associate tags to colors

The color is then used as background in the board, list and task details
views

tests/units/Formatter/TaskListFormatterTest.php

@@ -37,11 +37,13 @@ class TaskListFormatterTest extends Base
                 'id' => 1,
                 'name' => 'My tag 1',
                 'task_id' => 1,
+                'color_id' => null,
             ),
             array(
                 'id' => 2,
                 'name' => 'My tag 2',
                 'task_id' => 1,
+                'color_id' => null,
             ),
         );
 
@@ -52,10 +54,11 @@ class TaskListFormatterTest extends Base
                 'id' => 3,
                 'name' => 'My tag 3',
                 'task_id' => 2,
+                'color_id' => null,
             ),
         );
 
         $this->assertEquals($expected, $listing[1]['tags']);
         $this->assertEquals(array(), $listing[2]['tags']);
     }
-}
+}