Allow polymorphism in bulk record importer

A lot of EHRs don't follow standards very strictly. So I want to preprocess their files before sending them off to health data standards; it seems like the best way to do this is to let me create a class like

```
  class MyImporter < HealthDataStandards::Import::BulkRecordImporter
    def self.import(xml_data, provider_map = {})
      doc = self.fix_file(xml_data)
      super("", provider_map, doc)
    end
```

This pull request enables that by replacing internal calls to `BulkRecordImporter.X` with `self.X`.