Fix image references and continuation formatting

Fix image references and continuation formatting

This enables the production of epub output, as "a2x --format epub"
introduces stricter checking than the default formatting. Good for us,
we get to see more images and cleaner output!

Classes of fixes include:

* Leading '+' to keep numbered bullet sequences in sync only works if
  there are no blank lines around the '+' lines. Easiest to just keep
  the bullets with no blank lines between them.
* Typos in image file names.
* Intradocument link syntax (just use <<id,description>>)
* image:: entries like to have a leading blank line

Signed-off-by: Dan Scott <dscott@laurentian.ca>