Add importmtimes plugin for preserving file mtimes during copy imports.

File modification times are stored as item.mtime, item.added and
album.added.
Record mtimes instead of pre-import paths.

This adds support for move imports.

Item and file mtime preservation is now configurable and disabled by
default.
Only delete album item mtimes that are in the dictionary.
Fix style errors reported by Flake8.
Plugin rename importmtimes -> importadded.

This includes some minor changes to the documentation.
Merge pull request #520 from silb/importmtimes_plugin

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