Add exuberant-ctags to required packages to build and Debian as supported OS

-- added Debian as a supported OS alongside Ubuntu

-- added 'exuberant-ctags' to apt-get package list

-- added 'exuberant-ctags' to yum package list (untested, assuming
   same package name as apt)

ps. I'm still new to using github.  I'll get the commit message right next time :)