Created an Implementation of logback's Layout class so that users can log out to destinations other than a graylog2 server but output it in GELF.
Changed the Layout class to be instantiable and configurable via logback configuration.  It should match the GelfAppender now.

In order to share some methods that were originally in the Appender, they were pulled out into their own util class, and shared across the appender and layout classes.
Changed the Layout class to be instantiable and configurable via logback configuration.  It should match the GelfAppender now.

In order to share some methods that were originally in the Appender, they were pulled out into their own util class, and shared across the appender and layout classes.
Slf4j has the concept of markers, which allows users to mark log messages.  Logback currently implements this idea, but the gelf converter was ignoring the markers in the ILoggingEvent object that it had to convert.

I modified this code to have a flag called useMarker that tells the converter whether or not it should include markers in the GELF message.  This flag was added both to the GelfAppender and GelfLayout so that it could be passed through to the underlying GelfConverter.  I followed the pattern established by similar flags such as useLoggerName.
Only add the _marker element if useMarker is true AND the event actually has a marker.
Added the ability to put a newline between every gelf event in the GelfLayout.  I made this configurable to match how the standard logback json layout worked.

Before this change we were writing all gelf events out to the same line which was messing up grep-ability, and keeping things like logstash from working.
Added documentation to the readme about the additional useMarker configuration element.
Merge pull request #24 from solidjb/master

Added Logback Layout implementation and support for Logback Markers