Default model query changes

We can dynamically create the `default_model_query` for all service classes in the `BaseService` instead of defining it over and over again in the subclasses.

This removes a lot of semi-duplicate code and makes the codebase more DRY.

### Test Results:

```bash
 |2.0.0-p247| Marks-MacBook-Pro in ~/quickbooks-ruby
± |default-model-query ✗| → rake spec
Finished in 1.14 seconds
187 examples, 0 failures
Coverage report generated for RSpec to /Users/mrickert/quickbooks-ruby/coverage. 1740 / 1813 LOC (95.97%) covered.
```