(new): add log level helpers and HTTP method levels

- move LogLevel and predefined levels into levels.go
- add level-specific constructors for info/warn/error/fatal/debug
- add predefined HTTP method log levels and LogLevelForMethod
- unify Logger Print/Println internals and add Printf
- improve GoDoc coverage for exported APIs

time_format.go

@@ -2,6 +2,7 @@ package sneklog
 
 import "strings"
 
+// Predefined strftime-like time layouts.
 const (
 	Layout   = "%m/%d %I:%M:%S%p '%y %z"
 	ANSIC    = "%a %b %e %H:%M:%S %Y"