Issue #613 - Produce webhelp flavour of docbook output, for debrief web-site
introduce the logo from the Debrief home page
override the default style sheet
minor changes, to specify the custom style sheet and to overwrite the default DocBook logo
force admon (note, tip) graphics to be used
ignore the compiled webhelp content
Merge pull request #625 from pecko/issue613

Issue #613 - Produce webhelp flavour of docbook output
Merge pull request #626 from debrief/issue613

Allow stylesheet over-rides, replace icon
use a local property value - to ensure it doesn't break any other DocBook builds