Using symbolic links for common elements

I changed several common files in the themes directory to use symbolic
links.  This way, I don't have to worry about making the same changes
in both themes.
Initial attempt to include a jmpress option for new 'engine' config

I've added the changes I *think* are going to be necessary to allow
jmpress *or* impress to be used.  I have to check this though.
More files for jmpress support

I forgot to commit these in the previous commit.
Documenting dependency on Python-markdown and jmpress.