Add the option of calling associations by their alias/name as String

I've read a lot of examples that said that we could use strings to include associated models, but it wasn't, and i dind't saw neither on sequelize docs. So i changed some lines to allow it, instead of doing `User.findAll({ include: [{ model: Tool, as: 'Instruments' }] })` we could do `User.findAll({ include: [ 'Instruments' ] })` (as association already was defined)

Example:
--------------

```
M.User
	.hasMany( M.UserEmail, {
		as: 'Emails',
	})
	.hasMany( M.UserPassword, {
		as: 'Passwords',
	})
	.hasMany( M.Task )

	.find({
		include: [
			'Emails',
			'Tasks',
			'Passwords',
		],
	})
```