Add original xmlindent.rb file by Emmanuel Oga

Source:
https://gist.github.com/EmmanuelOga/322025
http://emmanueloga.wordpress.com/2009/09/29/pretty-printing-xhtml-with-nokogiri-and-xslt/
Modify hashbang and remove extraneous require
Add comment with link to original xmlindent.rb
Rename xmlindent.rb and make executable