Add the ability to set the default Logger

Sometimes you want to have a default logger that is used
for all of your database connections; you can now set the
default logger using `gorm.SetDefaultLogger`.

logger.go

@@ -18,6 +18,11 @@ var (
 	numericPlaceHolderRegexp = regexp.MustCompile(`\$\d+`)
 )
 
+// SetDefaultLogger replaces the default Logger instance.
+func SetDefaultLogger(l Logger) {
+	defaultLogger = l
+}
+
 func isPrintable(s string) bool {
 	for _, r := range s {
 		if !unicode.IsPrint(r) {