STI support reindexing and searching

This commit removes previous change made in #163 fixing #190 and supersedes #194
It also adds reindexing support for STI tables and supersedes #197
These before and after hooks can be replaced with

```ruby
      before do
        searchable_subclass_model = Class.new(ASuperclassModel) do
          include PgSearch
          multisearchable :against => :content
        end
        stub_const("SearchableSubclassModel", searchable_subclass_model)
        stub_const("NonSearchableSubclassModel", Class.new(ASuperclassModel))
      end
```