Improve store attributes feature

There are 2 changes in store attributes feature:

- cast value of a serialized attribute to a specified type
```ruby
class User < ActiveRecord::Base
  store :profile
  store_accessor :profile, :name, :age, :use_gravatar
 
  def age=(value)
     value = ActiveRecord::ConnectionAdapters::Column.value_to_integer(value)
     super
  end

  def use_gravatar=(value)
     value = ActiveRecord::ConnectionAdapters::Column.value_to_boolean(value)
     super
  end
end

# replaced by

class User < ActiveRecord::Base
   store :profile
   store_accessor :profile, :name, :age => :integer, :use_gravatar => :boolean
end
```

- default value
```ruby
class Car < ActiveRecord::Base
   store :settings
   store_accessor :settings, :color
   
  def color
     super || 'red'
  end
end

# replaced by 
class Car < ActiveRecord::Base
  store :settings
  store_accessor :settings, :color => { :default => 'red' }
end
```

2 features together

```ruby
class User < ActiveRecord::Base
  store :profile
  store_accessor :profile, :age => { :type => :integer, :default => 18 }
end
```

Also, `read_stored_attribute` and `write_stored_attribute` were changed. They don't need to specify store_attribute if it was specified in `store_accessor`.