New add_counter_cache_column to ActiveRecord migrations

Hi everyone,

I submitted a patch that creates a new add_counter_cache_column method
available to migrations. Apart from creating the <association>_count
column for you (with default 0), this will update the counter with the
current value for each row. So:

def self.up
  add_counter_cache_column :posts, :comments

Will generate the comments_count column on posts, plus give it initial
values, just like doing:

Post.find(:all).each do |post|
  Post.update_counters, :comments_count => post.comments.count

I found adding these columns is a very common practice when tuning for
performance as development evolves. Updating the counters with their
initial value seems like a must to prevent data corruption (imagine
your tests breaking after creating the magic column -- everyone
calling #size on the association suddenly gets 0).

Small doc update and tests: