support tmux 1.6

- default capture_args with -J only if supported (tmux 1.7+)
- tmux 1.6 does not support -p (send to stdout), so work with
  paste-buffers instead.
Use tmux capture-pane -p if available
Extract tmuxcomplete#init, reuse default s:capture_args
Update version to 0.0.4
Merge pull request #35 from wellle/justinmk-tmux1.6

#34 Support for tmux 1.6