expose debug as default export

+ Similar to tj/co/blob/master/index.js
+ Allow to re-export debug from another module in ES6

Author: Jorge Bucaran

debug.js

@@ -6,7 +6,7 @@
  * Expose `debug()` as the module.
  */
 
-exports = module.exports = debug;
+exports = module.exports = debug.debug = debug;
 exports.coerce = coerce;
 exports.disable = disable;
 exports.enable = enable;