added irb, ruby and _ruby

sheets/_ruby/hello

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+# to install: ruby is usually already installed
+#             if not, install it this way:
+#             (Debian/Ubuntu) apt-get install ruby
+#             (Fedora/CentOS) yum install ruby
+# to execute: ruby hello.rb
+puts "Hello world!"
+# or
+$stdout.puts "Hello world!"
+# or
+STDOUT.write "Hello world!\n"

sheets/irb

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+# interactive ruby. irb is a tool to execute interactively ruby expressions read from stdin
+
+# start irb
+irb
+

sheets/ruby

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+# Ruby is a dynamic, reflective, object-oriented, general-purpose programming language
+# ruby is a ruby interpreter
+
+# invoke Ruby from the command line to run the script foo.rb
+ruby foo.rb
+
+# pass code as an argument
+ruby -e 'puts "Hello world"'
+
+# The -n switch acts as though the code you pass to Ruby was wrapped in the following:
+# while gets
+#  # code here
+#  end
+ruby -ne 'puts $_' file.txt
+
+# The -p switch acts similarly to -n, in that it loops over each of the lines in the input
+# after your code has finished, it always prints the value of $_
+# Example: replace e with a
+echo "eats, shoots, and leaves" | ruby -pe '$_.gsub!("e", "a")'
+
+# BEGIN block executed before the loop
+echo "foo\nbar\nbaz" | ruby -ne 'BEGIN { i = 1 }; puts "#{i} #{$_}"; i += 1'