add kFpsRefreshRate constant
no need for constexpr here, compiler is smart enough
fix warning
add form_text & draw_fps functions
rename form{,at}_text
add private section to Hud
add draw_score
add update_score
update score within draw_score method
use boost::lexical_cast to convert num to string
format_text could be private
add draw_lives
update fps within draw_fps