Add minimal Doxygen support

Provide ART_BUILD_DOCS option which when true will recurse into the
doc/ subdirectory. Default ART_BUILD_DOCS to ON.

Implement minimal template Doxygen input file in doc/ subdirectory.
Provide CMake script that:

- Finds Doxygen (1.8 or higher)
- Configures template docygen input file
- Calls add_custom_command/target to wrap run of doxygen
- Installs generated HTML and tagfile to relevant directory

TODO: Provide cmake script to make tagfile available to clients via
find_package(Art).