JENKINS-21220 - Make XUnitProcessor serializable

When processing xUnit files on remote SSH slaves the following error
was obserserved:

 The plugin hasn't been performed correctly: remote file operation failed: /home/jenkins/workspace/xunit at hudson.remoting.Channel@56f87a20:slave

When investigating it was found that a NotSerializableException was
thrown. Simply make the needed classes serializable.
Merge pull request #16 from mindjiver/make-xunit-processor-serializable

JENKINS-21220 - Make XUnitProcessor serializable