Change to parent pom to gwt-dt

[kengu]

I'm almost ready to move gwt-leaflet to DISCOTools (see #14).

As preparation for this, most of gwt-leaflet parent pom is moved to gwt-dt. The change in gwt-leaflet is limited to removing most of the content in gwt-leaflet parent pom and change parent to org.discotools.gwt:gwt-dt.