Add ability to order by hstore columns in PostgreSQL

Example:

```ruby
class Device < ActiveRecord::Base
  store_accessor :information, :model
end
```

```ruby
ActiveAdmin.register Device do
  index do
    column :model, sortable: "information->'model'"
  end
end
```
Merge pull request #2844 from AndreyChernyh/f/add-hstore-sort

Add ability to order by hstore columns in PostgreSQL