POM cleanup.
Images are in a separate directory; see issue #37.
Build from Maven and Oxygen still works; a parameter (img.src.path) needed to be set in Oxygen.

--HG--
rename : src/main/docbook/front-matter/author.png => src/main/images/author.png
rename : src/main/docbook/front-matter/tombstone.png => src/main/images/tombstone.png