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.
Changed to not require collections

collections library is only available in Py 2.7+
Moved summarize_items to utils. Cleaned up output

Output now uses K for bitrate.
summarize function now in util
Pass found duplicates between task and session
Final reshuffle to put summarize_items in beets.ui

Also improved duration display.
Merge pull request #900 from howardjones/duplicate_album_hints

Basic summary information for duplicates
Formatting and style refinements for #900
Duplicate summaries work with singletons (#900)
Changelog for #900