automatic player discovery

[Yoni Rabkin]

In an effort to make Emms a better out-of-the-box experience, I've
pushed to the git repo the interactive function
`emms-setup-discover-players' in emms-setup.el.

`emms-setup-discover-players', if it works, interactively populates
`emms-player-list' based on binaries found on the system.

Please test it and comment so that we can ship it out as soon as
possible.

-- 
   "Cut your own wood and it will warm you twice"