Critical hit chance, knocking off balance.

Critical effects not yet implemented.

Clean up timed_effects.
First implementation of critical damage
Desperation attacks

Fix some bad effect description strings in recent work.  Off balance
properly prevents movement.
Start work on weapon critical effects.

Combat.iterCombat now returns object in addition to combat, combat_mult.  Add
Combat.findCombat.

Add Combat.isCrit and meleeTalent to clean up some duplicate code.  Rename
damage_message_passive to is_passive.  Update some comments.

Other tweaks:  Let never_move creatures be knocked down and knocked back.
Various cleanup to recent work
Spinning Halberd
Dual Strike, Bloodletting Strike
Blurring Blades Style

Tooltips for critical effects.

Other misc fixes.
Add critical effects to Off-Hand Attack

Fix mult for Sweep.
Merge branch 'crits'