refactor Hooks mixin

The existing Hooks mixin explicitly defines methods for all hooks on
the Sequelize instance or a given Model. This modifies the Hooks module
to provide a single method "applyTo", which mixes in the base Hook
functionality (addHook, runHooks, etc), as well as autogenerates
declarative hooks for the object its mixing in to (Model.beforeCreate,
Sequelize.beforeDefine, etc).

lib/model.js

@@ -2179,7 +2179,7 @@ module.exports = (function() {
   };
 
   Utils._.extend(Model.prototype, associationsMixin);
-  Utils._.extend(Model.prototype, Hooks);
+  Hooks.applyTo(Model);
 
   return Model;
 })();

lib/sequelize.js

@@ -268,8 +268,7 @@ module.exports = (function() {
    * Allow hooks to be defined on Sequelize + on sequelize instance as universal hooks to run on all models
    * and on Sequelize/sequelize methods e.g. Sequelize(), Sequelize#define()
    */
-  Utils._.extend(Sequelize, Hooks);
-  Utils._.extend(Sequelize.prototype, Hooks);
+  Hooks.applyTo(Sequelize);
 
   /**
    * A general error class