Added Logback Layout implementation and support for Logback Markers

We are not currently using graylog, but we are wanting to start processing our logs in Hadoop.  We wanted to make our logs parseable and found GELF in our research.  We may be integrating graylog in the future, but we wanted to make the change to parseable logs now.  So we need to produce GELF format in a way that can go out into any Appender.  I simply created an implementation of Layout that uses your GELFConverter class to create the GELF message.

Also, Logback has the concept of Markers in the ILoggingEvent class.  These markers are just extra information about the message.  I added support for getting these markers into the GELF message with a key of '_marker' so that they can be used in the parseable message.

Updated the readme with the useMarker documentation.