Users view current challenges and workouts

![image](https://cloud.githubusercontent.com/assets/8655639/5191651/2b922374-74be-11e4-8875-2d6be8480c2d.png)

better to refactor as:

```ruby
<ul>
    <%= render @challenges %>
</ul>
```

```ruby (app/views/challenges/_challenge.html.erb)
<li>
    <%= link_to challenge.title, challenge %>
</li>
```