Added "ad" and "audio" placeholders, cq-block-placeholder CSS

**Moved widgets javascript `widgets/source` to `widgets/source/js` folder**

Simple PR to add 2 new cq-placeholder icons (ad and audio), and `cq-block-placeholder` styles.

![english](https://f.cloud.github.com/assets/1451868/2405628/3caf2e88-aa55-11e3-9346-211a3da44d7a.png)

Above rendered with:
```
<img class="cq-audio-placeholder cq-block-sm-placeholder" src="/etc/designs/default/0.gif"/>
<img class="cq-ad-placeholder cq-block-placeholder" src="/etc/designs/default/0.gif"/>
<img class="cq-carousel-placeholder cq-block-lg-placeholder" src="/etc/designs/default/0.gif"/>
```

Its very fast to make new cq-placeholder icons for custom components, and they provide nice visual cues and add polish. The idea is to continue contributing adding new icons to this list.

`cq-block-*-placeholder` expands the border to 100% width, and 3 preset heights. This adds significant composure and polish to unconfigured pages; rather than the default of having a 64x64 inline image with a dotted border.

Naming is of CSS classes in the "standard" CQ naming format for consistency.