Use different callback names for followers/followables

This is something for models that acts as followers and followables.

### Before

```ruby
class User < ActiveRecord::Base
  acts_as_follower
  acts_as_followable

  after_follow do
    # This is triggered for both users!
  end
end

User.find(1).follow(User.find(2))
```

### After

```ruby
class User < ActiveRecord::Base
  acts_as_follower
  acts_as_followable

  after_follow do
    # This is triggered for the first user only
  end

  after_being_followed do
    # This is triggered for the second user only
  end
end

User.find(1).follow(User.find(2))
```