Issue 97: WYSIWYG

This includes a lightweight implementation of a contentEditable rich text editor. The rich text formatting commands available include:

 * bold
 * italics
 * underline
 * strike-through
 * link
 * unlink
 * remove formatting

Images can be added to the rich text area the same way they can be added to the plain textarea, just by dragging an image from the desktop onto the form.

The rich text capability can be disabled by filtering `liveblog_rich_text_editing_allowed` to be false.

Fixes Automattic/liveblog#97

![screen shot 2013-07-08 at 9 34 09 pm](https://f.cloud.github.com/assets/134745/766134/7a0f9508-e852-11e2-946d-035512c5f199.png)

![screen shot 2013-07-08 at 9 34 18 pm](https://f.cloud.github.com/assets/134745/766135/7e8daeda-e852-11e2-9acc-a9cf351fa16b.png)

![screen shot 2013-07-08 at 9 44 18 pm](https://f.cloud.github.com/assets/134745/766137/830aefb8-e852-11e2-8851-228445a25b91.png)

![screen shot 2013-07-08 at 9 44 37 pm](https://f.cloud.github.com/assets/134745/766138/8701e022-e852-11e2-990d-2314fd367c6c.png)

![screen shot 2013-07-08 at 9 45 18 pm](https://f.cloud.github.com/assets/134745/766139/8cf814e2-e852-11e2-934d-66f4abf8cec5.png)