refactoring com_content as com_portfolio

^ 'Content' => 'Portfolio'
^ content.php => portfolio.php
^ com_content
=> com_portfolio
^ COM_CONTENT => COM_PORTFOLIO
^ __content_frontpage =>
__portfolio_featured
__content_rating => __portfolio_rating
__content =>
__portfolio
content_id => portfolio_id
^ comment site lang for unused
stings
^ comment admin lang for unused stings