Add a plugin for preserving file modification times during copy imports.

My existing music collection contains information about when I added music into the collection in the files' modified times. I don't want to loose this information if I import my collection into Beets.

This plugin will preserve file modification times and also store them in the 'added' and 'mtime' fields in the Beets database.

Please review the code and let me know if anything should be changed.