[Code Style] Minor remark and style.
[API] Proper verbose logger initialization.

Now it's not possible to initialize verbose logger with garbage level value, because user specific severity enumeration may start with any value.
Instead of this the User if forced to provide severity threshold explicitly.

This is highly breaking change, but it's necessary.
[API] Make verbose logger constructor explicit.
[Code Style] Tiny functional decomposition.
[Planning] Changed description.
[Code Clean] Refactoring.

Using inner class for default filter instead of pair function.
[Planning] Oops, I've fucked up.
[Planning] More.
[Planning] Another planning stuff.