[Ann] Updates to org-drill (org topics as interactive "flashcards" using spaced repetition)

[Paul Sexton <psexton@xnet.co.nz>]

Org-Drill has recently been added to the contrib directory of the org repository.

Latest version is in repository at:
http://bitbucket.org/eeeickythump/org-drill

I have made a couple of major updates recently. Changelogs are below.
Reports of user experiences are welcome.

Version 1.0
Added README with more detailed documentation.
Items which are failed during session are presented again before the session 
ends.
Items which were failures at the last review session are presented first in the
next session.
Added recognition of leech items, based on excessive number of failures.
Leeches can be skipped or warned about.
When reviewing an item, the prompt shows the number of items remaining.
When the session finishes, the minibuffer shows a brief report givisng
statistics about total time spent reviewing, number of cards that
still need review, etc.
Fixed the regexp for cloze-deleted text.
Optional face for cloze text in org mode buffers.
New card type: twosided.

Version 1.1
Added implementation of SM2 algorithm (now supports both SM2 and SM5).
Add option 'org-drill-spaced-repetition-algorithm': choose either SM2 or SM5
algorithm.
Add option 'org-drill-add-random-noise-to-intervals-p': randomly vary repetition
intervals slightly, to avoid clumping.
Fixed a bug in org-learn's SM5 algorithm (intervals much too long).
Use overlays to display cloze deletions as '[...]' during reviews.
Cloze text can contain hints, which will visible during review.
Add option 'org-drill-failure-quality': customise which quality is regarded as
unambiguous failure (1 or 2).
Expanded documentation, and added section on 'incremental reading', with example
setup.