Support for redcarpet 2.2+ and kramdown

[al3xandru]

• Added support for redcarpet 2.2 (changed the API breaking process_markdown)
• Added support for kramdown

[leehambley]

Nice pull req. I had to work around by forcing 1.7.2 of redcarpet.

[rickhull]

Confirming the issue with recent versions of redcarpet. Please apply this to the mainline gem.

$ rocco foo.rb
WARNING: Pygments not found. Using webservice.
pygmentize not in PATH; using pygments.appspot.com instead
/usr/lib/ruby/gems/1.9.1/gems/rocco-0.8.2/lib/rocco.rb:447:in `process_markdown': uninitialized constant Rocco::Markdown (NameError)