Using direct keyword stringify substitution in log macro, i.e. LOG("level=%s", keyword::severity<level>).
Autoflush property for file sink can now be configured.
Several tests have been added.
Code cleaning.

Removed extra typedefs and methods.
Added forgotten include.
Massive refactoring done.

Sink and formatter factories are no longer singletons. Moreover they are renamed to group and fronted factories respectively.
New sinks and formatters cannot be added separately, but you can specify 1:1 or 1:m where m - is formatters each addition.
Now group frontend factory throws exception when requested non-registered sink.