Bug #911 reimport detection for the `importadded` plugin

Reimports are now detected by inspecting the fields for reimported items
and replaced albums in ImportTask.

Reimported albums and items are skipped.
Comply with PEP8.
Add quoting and use displayable paths in log messages
Merge pull request #925 from silb/bug-911-importadded-reimport

Fixes bug #911