Added gradle build and citedownutils package

This fork adds a grade build to the project that should not (?) interfere with the existing sbt build.  The only conflict I could foresee is that I renamed the src/main/java branch src/main/groovy to permit mixed comiplation of java and groovy source with gradle.  I haven't tested sbt with this configuration.

I added a citedownutils package alongside the citedown package.  It includes a utility for converting citedown to pure markdown by resolivng URLs to URNs according to some configured value or values of CITE service URLs. The implementation follows your suggestion of working directly with the parsed result of parsing with the CITEDOWN extension, rather than rewriting a whole new parser.

The implementation does not yet include every Node type that the parser can emit, but I'm working through that, gradually adding unit tests for each Node type.  Right now, it recognizes the most important elements of markdown:  paragraphs, headers, lists and reference lists at the block level, emphasis and strong at the inline level, and of course CITE reference links.

I put in a docs directory with some brief explanation and examples of what the cd->md converter does.