Sequelize.prototype.Sequelize

This PR adds `Sequelize` to `Sequelize.prototype`.

This may seem like a bit of a strange one. Why would you need access to `Sequelize` from `Sequelize.prototype` when you can just `require('sequelize')`?

I'm finding while writing plugins that I am listing Sequelize as a peerDependency rather than a straight dependency, so that you can chain multiple plugins like:

    var Sequelize = require('sequelize');
    require('sequelize-plugin1')(Sequelize);
    require('sequelize-plugin2')(Sequelize);

In a hook function in a plugin, you may need to access `Sequelize` to get to for example `Sequelize.Error` or some of the data types. In this case you don't want to `require('sequelize')` as you want to make sure that you get the instance of `Sequelize` that was passed to the plugin, so better to be able to use `sequelize.Sequelize`.

I don't know if that makes sense!

I guess it does clutter the API a little, but doesn't affect the tests or BC, and I think it could be useful in some cases. What do you think?