Replace <br /> with \n in Notifictation messages

* <br /> is not supported by all WMs
* \n has broader support or will be sliently (i.e. without printing a
  character) ignored
Move markup out of album string in D-Bus message.
Extract markup out of now playing message
Merge pull request #178 from xhochy/fix/dbus-notify-linebreak

Replace <br /> with \n in Notifictation messages