Basic summary information for duplicates

Added a one-line summary of each album (in lib, and import target) so
you can easily tell if (for example) you are about to overwrite your
FLAC copy with a low-bitrate mp3 from somewhere else.
Here's where I got lost in the structure of beets. The only place I could find that called task.find_duplicates() was in a multiple-clause if statement that threw away the return value. I was thinking in terms of keeping the changes within the task, but I can see what you mean about testing, too. Is there an intention to have different types of session (e.g. WebImportSession) in the future? I've reshuffled to remove the field.