Docs generate anchors for headers

Each header has now id (which [works as linkable anchor](http://stackoverflow.com/a/484781/531342)).
Nested headers have their ancestors in the id, but `h1` is ignored, because that's title of the page and so it's already contained in the url.

```html
<h1>$http</h1>
<h2 id="description">Description</h2>
<h3 id="description_general-usage">General usage</h2>
<h2 id="methods">Methods</h2>
<h3 id="methods_post">post</h3>
<h4 id="methods_post_parameters">Parameters</h4>
```

It also reports broken links:
```
WARNING: guide/directive (defined in docs/content/guide/directive.ngdoc) points to a non existing anchor "guide/directive#attributes"!
```

It also collects anchors, so it is possible to use `<a name="xxx"></a>` in the markup and then link to it and it won't be reported as a broken link.