Fix markdown separator in doc.
Also change the markdown separator in README.md.

This fixes issue #2.
Treat the slide buffer as scratch buffer.

You don't get a warning when you close the presentation with :q (instead of
the proper q) because the buffer is not associated with a file.
Explicitly set the filetype of PresentingDemo.markdown.
Fix issue #4.

The if clause for confirming the ft was not necessary, and, more
importantly, was not valid vim script.