[FEATURE] Add stat packages

...main/java/net/sourceforge/plantuml/pstat/LogCallback.java

...in/java/net/sourceforge/plantuml/pstat/StatSxServlet.java

src/main/java/net/sourceforge/plantuml/pstat/Stats.java

...va/net/sourceforge/plantuml/pstat/WriterOutputStream.java

.../java/net/sourceforge/plantuml/pstat/event/StatEvent.java

...urceforge/plantuml/pstat/event/StatEventAdminSaveLog.java

...sourceforge/plantuml/pstat/event/StatEventAdminStart.java

.../sourceforge/plantuml/pstat/event/StatEventAdminStop.java

...urceforge/plantuml/pstat/event/StatEventFileIterator.java

...rge/plantuml/pstat/event/StatEventFileIteratorSimple.java

...eforge/plantuml/pstat/event/StatEventFileIteratorZip.java

...sourceforge/plantuml/pstat/event/StatEventHtmlCreate.java

...rceforge/plantuml/pstat/event/StatEventHtmlLoadImage.java

...urceforge/plantuml/pstat/event/StatEventHtmlLoadPage.java

...eforge/plantuml/pstat/event/StatEventImageGeneration.java

...n/java/net/sourceforge/plantuml/pstat/event/UAParser.java

...main/java/net/sourceforge/plantuml/pstat/graph/Chart.java

...a/net/sourceforge/plantuml/pstat/graph/ChartProducer.java

...main/java/net/sourceforge/plantuml/pstat/graph/Curve.java

.../java/net/sourceforge/plantuml/pstat/graph/HistoList.java