Added 'report_untouched_recipes' coverage option.

This displays a list of recipes that are not covered at all by the specs. To use this feature:

```ruby
#Enable recipe reporting:
ChefSpec::Coverage.start! do
  report_untouched_recipes
end

#Disable recipe reporting:
ChefSpec::Coverage.start! do
  report_untouched_recipes false
end
```

The new coverage report looks like this:
```
ChefSpec Coverage report generated...

  Resource Total:    11
  Resources Touched: 10
  Resource Coverage: 90.91%

  Recipe Total:      5
  Recipes Touched:   3
  Recipe Coverage:   60.0%

Untouched Resources:

  package[test]                      /cookbooks/my_cookbook/recipes/default.rb:1

Untouched Recipes:

  /cookbooks/my_cookbook/recipes/another_recipe.rb
  /cookbooks/another_cookbook/recipes/default.rb
```

Unfortunately, the untouched recipe list does not respect the filters, because they are specifically designed to work only with resources.