Adding hidden htmllint task

Adding a Grunt htmllint task to help find possible HTML errors in our templates.

```sh
$ grunt htmllint
Running "htmllint:all" (htmllint) task
>> 10 files lint free.

Done, without errors.


Execution Time (2014-07-24 21:31:52 UTC)
loading tasks  1.4s  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇ 42%
htmllint:all     2s  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇ 58%
Total 3.5s
```
The one about _"Text not allowed in XHTML element "ul" in this context."_ is about it not liking the curly braces in the templates and interpreting those as random text blocks:

For example,
```html
<section>
  <ul>
    {{#devices}}
      {{#isCurrentDevice}}
        <li class="selected"><span>{{name}}</span></li>
      {{/isCurrentDevice}}
      {{^isCurrentDevice}}
        <li><a href="#devices/{{id}}">{{name}}</a></li>
      {{/isCurrentDevice}}
    {{/devices}}
  </ul>
</section>
```