(Issue550) Fix restoring preview items from revision

Copies the PreviewItems data from the previous revision when restoring a
revision.

No longer stores previews and content in timestamp-spaced directories,
instead revision directories. Should help exploding disk-space when
trying to fix / trobleshoot issues.

Add some additional tests for verifying small / medium images on other
document types.
(Issue550) Self review
Change name of 'copyRevision' property
Merge pull request #550 from mrvisser/issue-550

Restored revision has no preview items