Fix Debian install instructions and add tests

I've updated the readme to get it to install on Debian Wheezy, although it doesn't actually play mp3 files correctly - they play at about the right sound level, but the audio is all garbled.

I've also added some very basic rspec tests for this, really just as a way of encouraging others to write more tests.  I'm not testing any more than this can actually load an mp3 file and report its length.