Change API

## Background
```ruby
class Tweet < ActiveRecord::Base
  has_many  :replies, class_name: 'Tweet', foreign_key: :in_reply_to_tweet_id
  has_count :replies, class_name: 'Tweet', foreign_key: :in_reply_to_tweet_id
end
```

Because ActiveRecord association is under active develpoment, it would be hard to maintain a compatibility if I had overridden `has_many` association. So I designed this gem not to override `has_many` association.

But `has_count` association depends on `has_many` association existence and they should have the same options. Thus the current way to define `has_count` association is too lengthy.

## New API
```ruby
class Tweet < ActiveRecord::Base
  has_many :replies, count_loader: true, class_name: 'Tweet', foreign_key: :in_reply_to_tweet_id
end
```

New version will provide `:count_loader` option, which is similar to `:counter_cache` option. The option will define `CountLoader` association named `:replies_count`, and you can change association name if symbol is given instead of `true`.

So I'll do the API migration and rename gem to `activerecord-count_loader`.