fix build
add missing documentation
allow empty set of promised attributes

currentsong returns no attributes if current playlist is empty.
.travis.yml: add
.travis.yml: use sudo
.travis.yml: apt-get update to fix 404 errors
.travis.yml: try to stop mpd from listening on ipv6

travis is not allowing ipv6 listens?
test: better detection of mpd server
.travis.yml: use unix domain socket
.travis.yml: give up on running tests
README: add build status

--HG--
rename : README => README.md
README.md: fix build status url