Adding a constant for TIME_START as $_SERVER[REQUEST_TIME] is only

accurate to the second.  Profilers and timers need ms resolution
to be accurate.

lib/Cake/bootstrap.php

@@ -18,6 +18,8 @@
  * @since         CakePHP(tm) v 0.2.9
  * @license       MIT License (http://www.opensource.org/licenses/mit-license.php)
  */
+define('TIME_START', microtime(true));
+
 if (!defined('E_DEPRECATED')) {
 	define('E_DEPRECATED', 8192);
 }