Added support for two markup engines

There are now two parsers available.  I had to rename the source files
to avoid confusion between the parsers and the Python modules they use.

Now the user can specify the `markup` parameter as either `mikasa` or
`markdown`.  These are then the names of the respective Python
modules that will be used.

The advantage of using the `markdown` module is that it has nice
integration with Pygents which means syntax highlighting is quite nice.

There is a lingering issue which is that if you use the `markdown`
module, any `----` sequences get converted to `<hr/>` elements which
seems to confuse the code that (somewhere) cuts the HTML content
into slides.  This should also be a problem for `misaka` as well
but doesn't appear to be (which implies there is a bug in `misaka`).

I'll look into that next.
Fixed splitting issue

When switching between misaka and markdown processors, I noticed that the
slides didn't get split properly.  The issue was that the logical in
slides.py assumed HTML.  But markdown was spitting out XHTML.  The
difference was in the handling of the horizontal rule tag (and not
because of a bug in misaka as I earlier suspected).  So now the
splitting is robust whether the element is written as '<hr>', '<hr/>'
or '<hr />'.