Start introducing Enlive

Constructing HTML in the code was more a joke from the beginning. Remove
that and start with Enlive.
Serve pictures again

My handmade HTML generation facility generated a page with pictures. In
the course of introducing Enlive I had only generated a test page. Add
the code for generating a page with pictures again. Thus restore the
original functionality, but with Enlive.
REFACTOR Clean up some minor things in core.clj

Of course all the stuff that's in core.clj now shouldn't stay there. But
I'm experimenting and it isn't in the way, so leave it there for now.
REFACTOR Move normal-file lister out to utils

In two places I had code filtering out the non-directory files
from file-seq. Put it into a utils file, so that it's sensibly
accessible from everywhere.
Merge branch 'intro-enlive'