Make logging extensible

Allow developers to specify a logging class that adheres to interface `Hybrid_Loggers_iLogger` in the HybridAuth config(variable `logger`). This will fallback to the default behavior if nothing is specified.

The only potentially compatibility issue is that I have removed the `__construct` method from `Hybrid_Logger` itself, which will defer exceptions, and removed the `$logger` static variable from `Hybrid_Auth`(which was unreferenced due to all of the `Hybrid_Logger` methods being static.