Changes necesary for JRuby.

modified:   Gemfile
  Added logic for ruby vs jruby
    therhinoracer for v8
    kramdown as markdown engine
    sinatra-contrib for reloading
  Added markdown for seemless render

deleted:    Gemfile.lock
  For multi ruby runtimes, committing lock file is a tough one.

modified:   README.md
  Added steps for jruby

modified:   lib/commonplace.rb
  Swap in Markdown

modified:   lib/server.rb
  Check for jruby, and setup Sinatra:Reloader