Revised informalfigure handling

Revised informalfigure handling in accordance with HTMLBook spec:

* Don't generate `<figure>` or `<figcaption>` elements for informalfigure; just convert as `<img>`
* Also removed propagation of `float` attribute to `<figure>` element, as that attribute is not part of the HTML5 spec. (We may want to consider propagating float info as an inline CSS style)