would be nice if worked even in non Rails env

currently after :

```
require "unit_record"
ActiveRecord::Base.disconnect!
```

one gets :

  uninitialized constant ActiveRecord::ConnectionAdapters::UnitRecordAdapter::Rails (NameError)

cause of :

  unit_record-0.9.1/lib/active_record/connection_adapters/unit_record_adapter.rb:62
