Universal hooks for all models + hooks on Sequelize#define and Sequelize()

More hooks stuff! This PR...

#### 1. Allows hooks to be defined on all models universally

    sequelize.addHook('beforeCreate', function(...) {...});

This hook is then run on any model when a new instance is created.

#### 2. Creates hooks on `Sequelize.define` (`beforeDefine` and `afterDefine`)

#### 3. Creates hooks on `Sequelize()` (`beforeInstantiate` and `afterInstantiate`)

    Sequelize.addHook('beforeInstantiate', function(config, options) {
      config.database = 'db8';
    });

    var sequelize = new Sequelize('db1', 'user', 'password');
    // the database name is overriden by the hook

My use case for (3) is for a plugin to define universal hooks when it initializes:

    Sequelize.addHook('afterInstantiate', function(sequelize) {
      sequelize.addHook('beforeFind', function(options) {...});
    });

#### 4. Allows for hooks running sync for sync functions

This was neccesary to put hooks on `Sequelize#define` and `Sequelize()` as they are not async functions.
Done! Didn't know you could do it like that...