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)
As line 3 says, Red Cloth better be added to rmagick.gemspec as a development dependency, like this:

```
  s.add_development_dependency 'RedCloth'
```