Logger: extending should be allowed

Implementing a custom logger was essentially a suicide with theese methods private.

I need to collect all the messages that fall to tracy and effectively redirect them, without affecting the functionality. Yes, I really need tracy to **not write** any `*.log` files and redirect them somewhere else.

With theese methods `protected` and not `private`, I can now implement my own log method that does exactly what I need. 

If you know a better way how to achieve this, without extending the `Logger` class and without copy&paste of the code inside of the `Logger` class, please **do let me know**. Otherwise a merge would be really appreciated.

The third commit is optional, I don't need it critically, but it seemed as a nice feature. It's up to you @dg.

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use case: [tests/KdybyTests/Monolog/Extension.phpt](https://github.com/Kdyby/Monolog/blob/83aa41775e2c71530d829074acc007ee35ddfdc9/tests/KdybyTests/Monolog/Extension.phpt#L56)

with nette-2.1 [it works](https://travis-ci.org/Kdyby/Monolog/builds/34646381)
with nette-2.2 [it works](https://travis-ci.org/Kdyby/Monolog/jobs/34646410)
with nette-dev [it doesn't work](https://travis-ci.org/Kdyby/Monolog/jobs/34646405)