Now with ATI driver support (and bugfixes)

Changes from v1.1:

• Added support for steambox_ati: a SteamOS image with proprietary ATI/AMD drivers
• Fixed a bug in util/steambox that prevented some Pulse Audio sound from streaming
• util/steambox now prints an error if ${STEAMUSER_HOME} is unset
• Documentation improvements
• Dockerfile supportability improvements

Thanks to @jcorrado for bug fixes!