Make capistrano more 'rakeish'

As I started a discussion [here](https://github.com/capistrano/capistrano/pull/1043) I want to drive the discussion a little furthor. For myself I have not so many ideas but it feel's like that capistrano is still not so rakeish like it could be.

Additional to my thoughts in the pull request I would suggest as first step that capistrano can provide a load_additional_rake tasks method.
This one should replace the `Dir.glob('lib/capistrano/tasks/*.rake').each { |r| import r }` pattern in the Capfile. This way capistrano can ensure that additional tasks are loaded after all set statements in the defaults and stage file run.

Maybe it can be a default, when a folder lib/capistrano/tasks exists every task is loaded.