Make RDF an optional feature

[pombredanne]

There is quite a bit of dependencies and baggage that come with RDF support. I would like to make this an optional feature so we can have the rest working without RDF support

[goneall]

Just FYI - I just completed redesigning the Java tools to make RDF (as well as Tag/Value, JSON, YAML, and XML) optional formats. The project was restructured into a common library with a service provider interface for different serialization formats. There is a separate libraries for Tag/Value, RDF, JSON+YAML+XML, and Spreadsheet. This seems to have solved a similar issue of making the library lighter weight only bringing in the dependencies for the desired format.

The new Java library is here: https://github.com/spdx/Spdx-Java-Library