Fix logger base

#10

```ruby
Fluent::Logger::FluentLogger.open(CompoundFluentLogger, :host => 'localhost', :port => 24224)
```
In order to set up @@default_logger, I think that logger_base should not be extended.
Fluent::Logger.open should be used.

```ruby
Fluent::Logger.open(CompoundFluentLogger, :host => 'localhost', :port => 24224)
```

Now, `.open` of all inclusion Logger is an action which sets FluentLogger as @@default_logger and returns it.