Add feature spec for tag page (WIP)

* presents the tag name (pass)
* presents the tag description (pass)
* presents the links to the tagged people and projects (todo)
Improve factory for projects (sequence name attribute)
Add spec for TagPresenter (WIP)

* todo:
  move here expectations on generated html fragment from
  feature spec 'tag_page_spec'
Implement viewing list of tagged elements on tag page

* Introduces new context 'ViewingTaggables'
* Uses ViewingTaggables in TagPresenter with TagPresenter#taggables
Add gem simplecov
Fix spec for displaying detailed taggings on project and person page