Adjust the stat display system to use HTML custom data attributes.

* This replaces the old 'id' attribute based system, and is
more flexible, as the JS code will automatically find and update
any display in the HTML that follows the proper structure,
without needing to specifically code for each one.
Fix the zombie overpopulation bug.

* The zombie creation costs were badly broken; they had a fixed cost of
100
piety, and needed no housing or food. This made them way more efficient
than
humans during later stages of the game; so much more efficient that
while a
human civilization might have to stretch to reach 1M, zombie empires
could
easily reach 1B.
* This is corrected by using the same cost increase curve for zombies
that is
* used for humans. Costs are set at about 1/5 that of a human, because
piety
* is generated at about 1/5 the rate of food. This allow the player to
* essentially double their population with zombies, which brings this
power
* more in line with the other pantheon abilities.
Fix a bug that throws an exception as the game is resetting.
Fix a button enable/disable bug with the zombie buttons.
v1.1.10): Fix a bug in the food counter.

* It was displaying gross food gained instead of net food.