Update Ivy configuration and build script for integrated GWT.

Update Ivy configuration and build script for integrated GWT.

For ease of development, GWT "trunk" and "tools" are assumed to be
checked out in a directory parallel to the project directory. Otherwise,
the variable gwt.root can be used to give the location of trunk.

Add Ivy dependencies for servlet API and libraries needed to compile
tests (Jetty, Apache Commons) - these were previously used from GWT
JARs.