Add formatting options to LinkToHelper#simple_date

RE: #1358, #167 

Create `LinkToHelper#semantic_date` which returns
a html `<time>` tag including the correct attributes
and body

We could go ahead and use this, but it would be nicer
to use the existing `simple_date` helper. The only issue
here is that it is used in `.text.erb` templates, so we'd
have to include a format option or something, eg:

```ruby
def simple_date(date, format = :html)
  case format
  when :html then simple_date_html(date)
  when :text then simple_date_text(date)
  end
end
```