Use Kramdown on JRuby

There's a number of libraries that depend on rdiscount directly, and since there's no "markdown" meta-gem like there is for json, this commit makes a 'java' version of the rdiscount gem that just uses the pure-Ruby Kramdown library.

The rdiscount tests have very specific requirements about HTML formatting, so I wan't quite able to get everything to pass; markdown_test.rb fails 7 tests, and rdiscount_test.rb appears to hang (possibly a JRuby bug). But the output from Kramdown is usually correct "in spirit" if not exactly what rdiscount would produce.