Allow cleaning namespaces

[fatkodima]

Closes #58

This is still a WIP (needs tests and documentation). I need to verify from core members if this is a correct approach and based on the feedback will update the PR.

Consideration: when cleaning a namespace, should we clean also inner namespaces? Current implementation does so.

This PR will allow to do:

namespace :n do
  task :t do
    puts 'work'
  end
end

Rake::Namespace['n'].clear