Stage 1. Introducing more agile sink factory based on config.

Introduced file sink `Rotator` strategy.
File sink config is now template based on `Rotator` parameter.
Sinks now provide its `true name` and `parse` method to determine that name from config (boost::any ATM).
Examples temporarily disabled.
Tiny refactoring done over config parsing method which determines true sink name.

Rotator config is now vector.
File sink factory traits now usable at any Backend.
Config manipulation methods are moved into separate traits class.

Also renaming done.
File sink with rotation can now be properly initialized.
Tests review.

Proper naming and positioning.
Some comments added.
Exterminating magic constants.
All rotator stuff moved into separate file.
Enabling examples again.
Task for log rotation has been added.
Rotator changes.

Rotator structs are classes now. Absent includes added. Also minor wording.
File sink refactoring.

* There is no `null_rotator_t` class. Using template specialization instead of it.
* Reduced constructor number for `file::config_t`.
* Default values for rotator config.
* File sink has been refactored. The simplest file sink’s functionality has been splitted into writer and flusher.
* True sink name parser has been excluded from `config_traits`. Now it is free function, but not for long time.
Major renaming done.
Removed trivial (but unused) constructor for file sink.
Moving constant from function definition.
Rotator config now has initialization constructor with default values.
Rotator now depends on backend type (for universality).
Added file sink specialization with rotation.
Rotator template parameter is now template itself.

NoRotation tag added instead of `void`.