Enabling AsciiDoc/DocBook admonition icons

Copying AsciiDoc's admonition graphics into docs/static/images/icons
Add stub for automatically generated doc file
Adding javadoc assembly descriptor

This is used to unpack the titan-core javadoc jar for inclusion in
gh-pages during releases.  I should have included it in
7cab40ecd694b24a3ce8c6823e1f20a32071ff7c or
1ca0aed4f8be95bed9b26f3b2d464d042179e744, but missed it.