Adds support for counter cache in a Single Table Inheritance child model.

When we define a counter cache in the following way
```ruby
class Product < ActiveRecord::Base
end

class Review < ActiveRecord::Base
  belongs_to :product
end

class TwitterReview < Review
  counter_culture :product, column_name: 'twitter_reviews_count'
end
```
the `twitter_reviews_count` is updated fine as it is updated in callbacks of TwitterReview.

But 
```ruby
TwitterReview.counter_culture_fix_counts
```
does not consider `TwitterReview` to be a child in STI and returns the count of its parent.

I have updated the counts query to check if the current model is a STI child and add the `type` attribute condition like rails does in the JOIN clause.