Remove some Rubyforge references

Also I have some further questions:

1. As code documentation is written in YARD, do you agree that RDoc-related stuff in `yard.rake` can also be removed?
2. Do you want Addressable 3.0 to support ruby 1.8.7/ree? Or do you want to specify required_ruby_version as >= 1.9.3?
3. Probably there's no more reason to use `tasks/gem.rake` as rubyforge is dead - gemspec contents can be stored directly in `addressable.gemspec` and you can use `bundler/gem_tasks` to release.
4. Is website deployed somewhere? There are some unusual advices there (like `sudo gem install`, use submodules instead of `gem` with `:git` option). Probably website folder can be removed too.