Modified the build.xml file to make it easier to set up the configuration.
There is now a "config" target in which all of the properties that are usually
modified are located. Also, now one sets the location of tomcat and the
webapps dir, and chooses a name for the tomcat context, and everything else
is set relative to these. This means that only a few properties need to be
customized for a system (tomcat, webapps, context, username, password, and
the jdbc properties).
Finally, I also changed the pointer to xerces to use the locally present
version of the jar file, rather than an external one. Now the only external
jar file is the servlet.jar that ships with tomcat.