Add API version to Logger class

src/Monolog/Logger.php

@@ -76,6 +76,16 @@ class Logger implements LoggerInterface
      */
     const EMERGENCY = 600;
 
+    /**
+     * Monolog API version
+     *
+     * This is only bumped when API breaks are done and should
+     * follow the major version of the library
+     *
+     * @var int
+     */
+    const API = 1;
+
     protected static $levels = array(
         100 => 'DEBUG',
         200 => 'INFO',