add an image

for testing
add function to-abs-unix-path and fix slurp-bytes

Fix `slurp-bytes`...it didn't actually use its argument and always
looked for the same file (doh).  Add new function `to-abs-unix-path`
that takes a path-string and returns a absolute path resource.
Replaces system path delimeter (e.g. "\" on windows) with "/"
Replace two defresource lines with defresource-dir

Replaces the css script and image resource defenitions with a single
defresource-dir
add function defresource-dir

Add function defresource-dir that takes a directory path and, if it is a
directory, recursively adds all files beneath it (that are readable and
are regular files) to the resource map.