Added the Expenses app to the list of GWT samples, and modified each sample's build.xml file to set its source path

Review at http://gwt-code-reviews.appspot.com/752803

Review by: rice@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@8571 8db76d5a-ed1c-0410-87a9-c151d255dfc7