Remove build support for RAA and rubyforge; annotate Rake tasks

Added (commented out) RedCarpet to the Gemfile as it is used by
build_tarball.rake
to convert README.rc into README.html

build_tarball.rake
also builds and rewrites a bunch in doc
but doc doesn't go anywhere right now.

Re: updating the docs at http://www.imagemagick.org/RMagick/doc/
Tim Hunter, the original author writes:

  I wrote the original RMagick doc (http://www.imagemagick.org/RMagick/doc/) files by hand.
  No automatic generation... back when I was actively working on RMagick I usually communicated with
  John Cristy, the creator of ImageMagick. I suggest using
  the general ImageMagick contact page http://www.imagemagick.org/script/contact.php
  to send him a note, or post a note in the Developers forum on the ImageMagick discourse server
  http://www.imagemagick.org/discourse-server/

I'm not sure yet what to do with:

See https://github.com/rmagick/rmagick/wiki/Todo
Doxyfile # doxygen Doxyfile generates doc/c
README-Mac-OSX.txt # look at diff when moved to rubyforge?

These are almost certainly legacy
setup.rb
uninstall.rb
post-install.rb
post-clean.rb
post-setup.rb
metaconfig

See https://github.com/rmagick/rmagick/wiki/Release-Process
1. update ChangeLog
2. in  RMagick_git
  git tag RMagick_2-13-1
  git push origin tag RMagick_2-13-1
3. in RMagick_cvs
  # edit the username in build_tarball.rake to be you on line 18
  rake -f build_tarball.rake clean release=RMagick_2-13-1
  rake -f build_tarball.rake release=RMagick_2-13-1
  # should create all the zip/gem files you need
4. (dead) Upload the files as a new release to RubyForge, remove old
5. Update the RMagick website’s news section with information on the
release (in index.html)
Add releasing notes
Retire old build files
use pre-existing textile-based README
Update release tasks with legacy info
Break up release tasks
Merge pull request #69 from gemhome/update

Remove build support for RAA and rubyforge; annotate Rake tasks