1. Creation of parse trees works.
2. RFC 2396 seems to correctly draw parse trees for URIs.

* [GV_DCG,EXT] Added options argument to gv_tree//2. This allows one to set the options for export_ugraph/3.
* [GV_FILE,EXT] Added options argument to gv_tree//2. This allows one to set the options for export_ugraph/3.
* [RFC_2396,SC] Vastly simplified the arguments for all DCG rules. Now only the parse tree argument is used. Maybe we need some arguments later to return the most important URI components separately.
* [RFC_2396,NEW] Added uri_to_gv/1 which creates a visual representation of the parse tree for the given URI.
* [RFC_2396,DOC] Added an illustrative example of a parsed URI.
* [UGRAPH_EXPORT,EXT] Added several graph options to export_ugraph/3.