PDF docs build fails

[dihm]

After merging #70, discovered that mainline stable/latest builds on read the docs include building downloadable docs in the form of PDF and EPUB. Since I hadn't explicitly checked those builds before merging #70, naturally something failed.

After a fair bit of poking, discovered the problem is due to the default_value attribude of the Lightcrafter DMD ImageSet class being a raw bytes bitmap object that is crazy long. This causes and overflow error when building the associated latex.

The simple fix is to explicitly exclude display of that value for all docs.