Support for Markdown and jmpress.js

There are two major changes here.

The first is that I added support for Python-Markdown.  You can now see from the sample
presentation an immediate benefit because if Pygments is installed, it does syntax
highlighting on all the command-line code fragments in the sample presentation.  But it
also provides this feature for a **wide** range of languages.  Furthermore, I've turned on
some extensions in Python-Markdown that allow you to set the class and id and attributes
for elements generated from the markdown processor.  This gives you much finer control
over styling.  But it is also useful in conjunction with the second change.  The use of
the original misaka markdown processing or the Python-Markdown processor is controlled
by the `markup` option in `config.yaml`.  For misaka, use `misaka` and for
Python-Markdown use `markdown`.

I also added support from jmpress.js.  This is pretty much a drop in replacement for
impress.js but it adds several significant features.  The most important (for me) was
adding [animation support](http://jmpressjs.github.com/jmpress.js/examples/animation).

The choice of jmpress vs. impress is controlled by the `engine` option in `config.yaml`.

The animation support in jmpress requires that you add an attribute named
`data-jmpress` to the element you want to animate.  This can be done using the
`markdown` processor extension (automatically enabled with these changes)
[`attr_list`](http://packages.python.org/Markdown/extensions/attr_list.html).