Use print-as-a-function and absolute imports in all files.

This will:
- Be one less thing that needs to be changed for Python 3.x support.
- Ensure the development copy of lap won't accidentally import from an
  installed system copy.
- Prevent the "ui.urwid imports itself rather than urwid" bug that
  prompted it to be renamed to "ui.urwid_chooser" in the first place.
Fix some PyLint complaints in setup.py

(Prospector ignored it but Syntastic in vim has now been exposed to it)
It turns out that USE_PAGER is vestigial.

(Possibly something I intended to implement but never got around to)
Rename "backends" to "output"

(I've got other types of "backends" about to be split out)
Audacious removed the MPRIS /TrackList interface in 3.5.x. Acknowledged.