Additional fields in log msg via Java sys-prop

 * Adds support for additional, user-defined fields in JSONEventLayoutV1
 * These fields are passed by setting a system property. ie:
   -Dnet.logstash.log4j.JSONEventLayoutV1.UserFields=key1:val2[,key2:val2,key3:val3]
Merge pull request #27 from vrivellino/master

Additional fields in log msg via Java sys-prop