Plugin fails when run in Windows (\ instead of / for path sep)

[deors]

When the plug-in runs in a Windows box, it fails with an ArrayOutOfBoundsException. Root cause is in PitPublisher class, line 124, where it is expected that path separator is a forward slash. So, for example, when run in Windows, the report name is something like "C:\java\data\jenkins\jobs\test-mutation\workspace\target\pit-reports\201803261622\mutations.xml", thus no element with index 1 is obtained from the split("/"). Using the path separator for the platform might solve the issue.