Liquid tags

This adds a set of extensions to use Liquid-style tags within markdown (similar to those provided by octopress).  There are four tags included at this time, though the module is written to be easily extensible:

- Insert images of a given size:

        {% img /url/to/image.png [width] [height] [title] [alt]%}

- Insert HTML5/flash compatible videos:

        {% video /url/to/video.mp4 [width] [height] [title] %}

- Insert code from a file, with a title and link to download:

        {% include_code filename [title] %}

- Insert an HTML-rendered ipython notebook:

        {% notebook filename.ipynb [cells[start:end]] %}

Examples and details of how to use each are in the Readme file, and in the doc strings of each plugin.