Adapted to EXPath Application Package

Hi Chris - Great work on this graphviz application.  I got it working locally and thought I'd try to adapt it to the structure of an EXPath Application Package, to facilitate sharing and installation by others.  I added the required metadata files, as well as an ant build.xml file, which creates the .xar file.  Anyone can take this .xar file and install it with the eXist-db Dashboard's Package Manager.  I also made a few other minor modifications - to piggy back on the Dashboard's login page rather than hardcoding the admin password in the app.