fox for Logback-1008 - groovy configuration appenderRef for AppenderAttachable

fix for http://jira.qos.ch/browse/LOGBACK-1008

When you see ConfigurationDelegate.groovy's 136 line,
```
AppenderDelegate ad = clazz.name == 'ch.qos.logback.classic.AsyncAppender' ? new AsyncAppenderDelegate(appender, appenderList) : new AppenderDelegate(appender);
```
this line check the class is AsyncAppender.
But there is another async appender like reactor-logback's AsyncAppdener.

So this linke should changed to check if the class implements ch.qos.logback.core.spi.AppenderAttachable.

Because of this, we cannot use reactor-logback with groovy configuration.