Added LoggerAppenderGraylog2

Signed-off-by: Ivan Habunek <ihabunek@apache.org>
Added LoggerLayoutGelf

Signed-off-by: Ivan Habunek <ihabunek@apache.org>
Added documentation for LoggerLayoutGelf

Signed-off-by: Ivan Habunek <ihabunek@apache.org>
Added links in docs to new appender and layout

Signed-off-by: Ivan Habunek <ihabunek@apache.org>
Fixed indent in docs

Signed-off-by: Ivan Habunek <ihabunek@apache.org>