When using Rouge with Redcarpet, require 'rouge' is compulsory

`rouge/lexer` is loaded in `lib/rouge.rb`, not in `lib/rouge/plugins/redcarpet.rb`. Thus user would get the error `uninitialized constant Rouge::Plugins::Redcarpet::Lexer` if `require 'rouge'` is not included.
Merge pull request #110 from cmpitg/patch-1

When using Rouge with Redcarpet, require 'rouge' is compulsory