Implement pushing
Update desc, pushing now works
implement pushing of jellies

I wrote a much more complicated patch to do this, but Anatoly Vorobey
<avorobey@gmail.com> sent me a much simpler implementation before I had pushed
mine!  This patch is his algorithm with a bug fixed.
fix an animation glitch

If a push led to a fall, the fall could join before the fall animation
completed.
dynamically fill level picker
Revert "Update desc, pushing now works"

This reverts commit 1b4685c54a07ba9ea7fdb8648e749bea831e0e2d.
Revert "Implement pushing"

This reverts commit 9a28e68bd09858c47ebd452cbba26235095e8787.
Merge https://github.com/martine/jelly

 commit 9a28e68bd09858c47ebd452cbba26235095e8787.
Refactor:
- color a property of a cell, not a jelly.
- stage.cells mapping is x,y->cell, not x,y->jelly.
Refactoring: merging is driven by cells, not jellies. Introduce
immovable jellies.
Fix wall borders.
Support for anchors, both in the logic and in the UI.
Add 8th level
Add support for  black jellies and a few more levels.
Fix a subtle bug introduced by refactoring merges that didn't come
up until level 11. If as a result of the move two jellies begin
touching in more than one place, the first pair of touching cells
will unify the jellies and the second won't even call mergeWith,
so the borders will remain.

A bit ugly, but the only simple way I see of fixing is to allow merging
two cells within the same jelly, for proper handling of borders. To prevent
a huge number of null DOM updates, track directions in which each jellycell
already merged, and don't even try merging there.
A new level completion check - the old one is inadequate when one
jelly contains different colors, due to anchoring.
Some more levels, doc change.
Another subtle bug: when checking if a jelly can fall, we allow several to fall
together, but then actually move only one, which can corrupt @cells. This took
until level 12 to manifest.
Level 14 and a bugfix: now that we can mergeWith within the same
jelly due to a recent bugfix, don't count that towards victory.
All levels to 20.