Setup YardDoc

We had this in the Rakefile

#
# TODO: setup a place for documentation and then get this going again.
#
# begin
#   require 'yard'
#   DOC_FILES = ['lib/**/*.rb', 'README.markdown']
#   
#   YARD::Rake::YardocTask.new(:doc) do |t|
#     t.files   = DOC_FILES
#   end
#   
#   namespace :doc do
#     YARD::Rake::YardocTask.new(:pages) do |t|
#       t.files   = DOC_FILES
#       t.options = ['-o', '../grape.doc']
#     end
#     
#     namespace :pages do
#       desc 'Generate and publish YARD docs to GitHub pages.'
#       task :publish => ['doc:pages'] do
#         Dir.chdir(File.dirname(__FILE__) + '/../grape.doc') do
#           system("git add .")
#           system("git add -u")
#           system("git commit -m 'Generating docs for version #{version}.'")
#           system("git push origin gh-pages")
#         end
#       end
#     end
#   end
# rescue LoadError
#   puts "You need to install YARD."
# end

Review the commented YARD setup in the Rakefile, including DOC_FILES for lib/**/*.rb and README.markdown and the doc:pages publishing tasks. Check how the repository currently handles documentation, then restore a working generation and GitHub Pages publishing flow; done means the documented tasks run successfully and produce the expected docs.