added EXPath Package metadata files and a build file for creating the package (.xar)
reorganized repository to conform to structure of an EXPath Application Package
- now graphviz can be installed as an EXPath application package by running the build file and uploading the .xar via the eXist-db Dashboard's Package Manager app
- added basic URL rewriting so accessing /exist/apps/graphviz will redirect to the main index.xq page
- replaced xmldb:login() and hardcoded dba credentials with calls to the persistent login module
- corrected an apparent typo in the graphviz.xqm namespace uri (qraphviz > graphviz)