Migration helper functions

Provides support to migrations by adding the following methods:

* `install_hstore` - creates the hstore extension
* `uninstall_hstore` - drops the hstore extension
* `add_hstore_index` - creates a `:gist` or `:gin` index for an hstore column
* `remove_hstore_index` - drops a `:gist` or `:gin` index on an hstore column

Example schema migration for a products table to add an hstore index:

```ruby
class AddHstoreIndexToProducts < ActiveRecord::Migration
  def up
    add_hstore_index :products, :hstore_data, :type => :gin
  end

  def down
    remove_hstore_index :products, :hstore_data
  end
end
```