Create a {% figure %} tag

Creates a `<figure>` containing
  * an image with alt text
  * a link to a full size version of image
  * a `<figcaption>` based on content of tag
  * capability to render that content as markdown
  * CSS class hooks for size and orientation

Uses jsdom and jquery to strip out HTML from rendered markdown version of caption for use as alt text.

From
```
{% figure borneo-iphone-025.jpg landscape %}
A [barritonia](http://www.projectnoah.org/spottings/231146016 "Project Noah") flower
{% endfigure %}
```

Creates
![screen shot 2014-08-25 at 13 00 52](https://cloud.githubusercontent.com/assets/319055/4029431/8334f8d4-2c4f-11e4-8ed0-dff301e9f351.png)