Move from referencing the BlueCloth constant directly, to referencing Markdown.

This supports alternative implementations of markdown such as rpeg-markdown or rdiscount, and later releases of bluecloth.  There are some performance issues with earlier releases of bluecloth, and you should probably upgrade.  In the event that you can't you can place the following code into an initializer:

  Markdown = BlueCloth

actionpack/lib/action_view/helpers/text_helper.rb

@@ -271,8 +271,8 @@ module ActionView
       end
 
       # Returns the text with all the Markdown codes turned into HTML tags.
-      # <i>This method requires BlueCloth[http://www.deveiate.org/projects/BlueCloth]
-      # to be available</i>.
+      # <i>This method requires BlueCloth[http://www.deveiate.org/projects/BlueCloth] or another
+      # Markdown library to be installed.</i>.
       #
       # ==== Examples
       #   markdown("We are using __Markdown__ now!")
@@ -288,7 +288,7 @@ module ActionView
       #   markdown('![The ROR logo](http://rubyonrails.com/images/rails.png "Ruby on Rails")')
       #   # => '<p><img src="http://rubyonrails.com/images/rails.png" alt="The ROR logo" title="Ruby on Rails" /></p>'
       def markdown(text)
-        text.blank? ? "" : BlueCloth.new(text).to_html
+        text.blank? ? "" : Markdown.new(text).to_html
       end
 
       # Returns +text+ transformed into HTML using simple formatting rules.