Doxygen support added

Includes a doxygen config file, a mainpage.dox placeholder,
and a css file for making the pioneer badge.png not so huge.
gitignore doxygen generated documentation
COMPILING.txt: consistent section spacing, number sections, document
naturaldocs and doxygen
make game log font one step bigger (than FONT_SMALLEST)
Merge branch 'gamelog_bigger_font'
Merge branch 'doxygen'