Refactored logging

This PR changes the way sequelize logs SQL statements etc. Instead of always checking for options.logging, log statements are now just passed to `sequelize.log`. The following stuff is supported:

If `options.logging` is false => `sequelize.log` won't do anything.
If `options.logging` is a function which is not `console.log` => `sequelize.log` will pass the every but the first argument to the function.
If `options.logging` is `console.log` => use the winston logger and use the first argument as the event name (info, error, ...) and the other options as message(s). If only one argument is passed to `sequelize.log` use the info event.
If `options.logging` is an object => instantiate a winston logger with `options.logging` as options.