Remove build support for RAA and rubyforge; annotate Rake tasks

Added 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.

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)
I disagree.  My philosophy is that development dependencies in the gemspec should be what's required to run the tests, i.e. to develop the gem.  Anything else that is used in perhaps debugging it, documentation, etc. can go in the Gemfile.