Use AR::CounterCache#update_counters directly

Instead of using the ActiveRecord::CounterCache#increment_counter and #decrement_counter,
use the lower-level #update_counters method, because it will eventually enable us to
control the amount of the increment or decrement.  Otherwise, the amount is hard-coded
to 1 or -1.
Added the :delta_column option
Improved the description of totals tracking
Improved the README's totals example
Two code refactor; no functional change

In preparation for a bug fix in the next commit.
Fixed the handling of updated delta column values
Merge branch 'pr/20' Fixes #20

Conflicts:
	spec/counter_culture_spec.rb