Include ActionView::Helpers::TagHelper to the layout helper.

Including ActionView::Helpers::TagHelper to Object looks like bad practice. Object is polluted with helper methods. It introduces bugs if classes of an application have method names that conflict with the methods of ActionView::Helpers::TagHelper.

I encountered the issue when tested specs using FactoryGirl (see "tag" in code below):
```
factory :page do
    ignore do
      tag 'a tag'
    end
     
    tags { [tag] }
    ...
```