Add multi-threading support to Scanner

[postmodern]

Add multi-threading to Scanner. This can be done by using the ensure keyword in Scanner#each to stop all scanner threads.

Pseudo-code

class Scanner

  include Enumerable

  def each
    loop do
      yield rand(256)
      sleep 1
    end

  ensure
    puts 'stopping threads'
  end

end

scanner = Scanner.new
scanner.first(10)
# stopping threads
# => [30, 165, 245, 11, 43, 159, 13, 113, 128, 197]