Log Enhancements

Related to https://github.com/fuel/core/pull/1126

I think that would be a great addition to the framework some enhancements to log:

1. Changes Log to be Driver-Based
    * Makes possible to have logs stored on file or database simply changing on config (similar to session).
2. Makes easier to understand custom levels.
3. Creates an Event (log_called for example) so we can easily use that for example to send an e-mail if we get error log.

__Important Note:__ This change will not make any application crash, only will be need a heads up about configuration because it moves from config.php to a log.php file.

This is an initial commit so i can get feedback and see if it's getting good.

Still pending:

1. Makes an Oil Task for create the logs table as we have on session.
2. Change Docs

Let me know your thoughts.