Permit nested features

[tgaff]

In 2.1.0 using nested features such as:

feature 'big main feature' do
  feature 'sub feature' do
    scenario 'behaves well' do
       .......
    end
  end
end

results in a NoMethodError

/Users/dev/devel/capybara/spec/rspec/features_spec.rb:38:in `block in <top (required)>': undefined method `feature' for #<Class:0x007fb6c69049b0> (NoMethodError)
    from /Users/dev/.rvm/gems/ruby-1.9.3-p392@oceans/gems/rspec-core-2.14.1/lib/rspec/core/example_group.rb:245:in `module_eval'
    from /Users/dev/.rvm/gems/ruby-1.9.3-p392@oceans/gems/rspec-core-2.14.1/lib/rspec/core/example_group.rb:245:in `subclass'
    from /Users/dev/.rvm/gems/ruby-1.9.3-p392@oceans/gems/rspec-core-2.14.1/lib/rspec/core/example_group.rb:231:in `describe'
    from /Users/dev/.rvm/gems/ruby-1.9.3-p392@oceans/gems/rspec-core-2.14.1/lib/rspec/core/dsl.rb:18:in `describe'
    from /Users/dev/devel/capybara/lib/capybara/rspec/features.rb:22:in `feature'
    from /Users/dev/devel/capybara/spec/rspec/features_spec.rb:8:in `<top (required)>'
    from /Users/dev/.rvm/gems/ruby-1.9.3-p392@oceans/gems/rspec-core-2.14.1/lib/rspec/core/configuration.rb:896:in `load'
    from /Users/dev/.rvm/gems/ruby-1.9.3-p392@oceans/gems/rspec-core-2.14.1/lib/rspec/core/configuration.rb:896:in `block in load_spec_files'
    from /Users/dev/.rvm/gems/ruby-1.9.3-p392@oceans/gems/rspec-core-2.14.1/lib/rspec/core/configuration.rb:896:in `each'
    from /Users/dev/.rvm/gems/ruby-1.9.3-p392@oceans/gems/rspec-core-2.14.1/lib/rspec/core/configuration.rb:896:in `load_spec_files'
    from /Users/dev/.rvm/gems/ruby-1.9.3-p392@oceans/gems/rspec-core-2.14.1/lib/rspec/core/command_line.rb:22:in `run'
    from /Users/dev/.rvm/gems/ruby-1.9.3-p392@oceans/gems/rspec-core-2.14.1/lib/rspec/core/runner.rb:80:in `run'
    from /Users/dev/.rvm/gems/ruby-1.9.3-p392@oceans/gems/rspec-core-2.14.1/lib/rspec/core/runner.rb:17:in `block in autorun'

This might not the most common use-case but rspec does permit nested describes so capybara should probably behave similarly.

In the future if self.feature becomes more complex it might be better to move it to another module and then extend it instead.

[gaganawhad]

Curious: Any idea on when the next version of capybara might be available ?