Project level overrides

Here are three commits that add support for generating Idea project files, and allowing a project to provide image and css overrides for the HTML and PDF targets. Also tweaked the depends from `assembly` to `jar` so the docbook zip gets included on a `gradle install` for local SNAPSHOT testing.

Its not the most elegant solution, but its the least impact on the existing code. 

Probably would be nice to have an 'assemble' phase that can be tweaked from the build file so image and files can be copied into the working space before rendering. Time permitting i'll look at that along with overrides for the 'reference' target path for multiple tasks per project.