Update to GWT 2.4.0.

Update to GWT 2.4.0.

* .gitignore: Ignore new cache directory.
* .settings/com.google.gwt.eclipse.core.prefs: Update Eclipse settting.
* pom.xml: Change GWT and maven plugin to 2.4.0.
* src/main/java/org/glom/web/OnlineGlom.gwt.xml: Update doctype for
2.4.0.
* src/main/java/org/glom/web/client/ClientFactory.java:
* src/main/java/org/glom/web/client/ClientFactoryImpl.java:
* src/main/java/org/glom/web/client/OnlineGlom.java:
Update source for API changes.
* utils/build-onlineglom-war.sh: Remove cache directory before the
build.