header_links.html.erb

      <div class="header-link">
        <% if !user_signed_in? %>
        <%= link_to "Sign Up", new_user_registration_path %>
        <% end %>
      </div>
      <div class="header-link">
        <% if user_signed_in? %>
        <%= link_to "Sign Out", destroy_user_session_path, :method => :delete %>
        <% else %>
        <%= link_to "Sign In", new_user_session_path %>
        <% end %>
      </div>