CLS support in transactions

This PR adds support for automagically passing the current transaction to all queries within the callback passed to sequelize.transaction. For example:

```js
sequelize.transaction(function (t) {
  // Normally you'd have to pass { transaction: t }, but CLS handles that automagically
  User.find(where); 

  // To disable automatic passing, pass another transaction, or null for no transaction
  User.find(where, { transaction: t2 });
  User.find(where, { transaction: null });
});
```

This is still WIP - I'd like to add docs in this PR as well

Adresses https://github.com/sequelize/sequelize/issues/2717 and 

Closes https://github.com/sequelize/sequelize/issues/1627
Save the sequelize instance on the promise prototype, so `_then` can access the namespace. This is not ideal, since it does not allow different namespaces for different sequelize instances...