Further improvements to Mavenization

 * renamed core and sample to eventbinder and eventbinder-sample
 * simplification of the POMs (moving things to the parent POM to benefit
   from inheritance in the child POMs)
 * moved the sample packaging=war (gwt-app doesn't package src/main/webapp)

Also added inherits and source to the gwt.xml to be a better GWT-citizen.
Use dependencyManagement
Merge pull request #3 from tbroyer/maven

Further improvements to Mavenization