add doxygen to travis-ci build.
add doxygen to travis-ci build.

kinetiknz/nestegg,SingingTree/nestegg,SingingTree/nestegg,SingingTree/nestegg,bit/nestegg,bit/nestegg,fallaciousreasoning/nestegg,kinetiknz/nestegg,fallaciousreasoning/nestegg,kinetiknz/nestegg