Use pure Ruby Markdown parser for JRuby compatibility

When generating documentation, use kramdown instead of rdiscount to
parse Markdown. It is implemented in pure Ruby (no C extensions) so it
is compatible with JRuby and other Ruby interpreters. It is somewhat
slower on MRI but still reasonably fast.
Merge pull request #366 from sferik/kramdown

Use pure Ruby Markdown parser for JRuby compatibility