Clean up, specification and plans.
Basic syslog sink with its native backend has been implemented.
Syslog sink now has strong typed priority enumeration. Also it accepts enumerate type as template argument and requires user to provide mapper from its custom log severity to the syslog priority level.

Some tests has been written.
Drop tasks.
Hiding testing namespace.
Syslog frontend specialization has been implemented.

Functional test for syslog-based logger has been written.
Fix message scope level for functional test.
Forgotten includes, code style and plans.
Logger initialization factory has been implemented for popular combination of formatters/sinks.
Code style.
Tiny refactoring over frontend factory function.

Less type definitions, more templates.
One todo less.
Logging frontend factory functions is now static methods of `factory_t` class.

This change makes code a bit cleaner.
More plans.
Logger fronted factory has been moved to the separate file.
Attribute extracting class changed its location.

Log record class has own helper extraction function.
Styling.
Hiding std::underlying type under own wrapper.

This is done, because std::underlying_type supported only in clang and GCC 4.7+.
Tiny styling.