Issue 542 - Add small/medium preview images for PDF files.

Refactored the image handling in the PP slightly.
There is now 1 central utility which generates all the preview images
given an input image.
Actually download the preview images in the unit tests for the preview processor.
Merge pull request #542 from simong/issue-542-add-medium-image-to-pdf-previes

Revision history previews problem
(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
#553 - The `downloadPath` property should only be specified on content of resourceSubType `file`.
fix tests broken by DST
sort for consistent results
Using tenant default for visibility and joinability
Merge branch 'dsttests' of git://github.com/stuartf/Hilary into dsttests
Added a TODO for better timezone matching + documentation fixes
Merge branch 'master' into createGroupDefaults
Merge pull request #556 from nicolaasmatthijs/createGroupDefaults

Create group defaults
Merged master into issue-543-library-leaking