Provide gwt logging in playn-html.
Merge branch 'gwt-logging' of https://github.com/nordligulv/playn
Use correct package name.
Added file header.
Use name Gwt.gwt.xml.

No need to repeat logging thusly playn.logging.GwtLogging. Plus this matches
the pattern established by Enhanced.gwt.xml -> enhanced (directory):
Gwt.gwt.xml -> gwt (directory).