Added support for setting custom global model manager in settings using ...

...REPORT_BUILDER_MODEL_MANAGER and/or setting custom model manager per model using report_builder_model_manager property.

Note: 
Added two test one for the global manager settings and another for per model custom manager. For the per model test, I wanted to ensure coverage of my changes to get_query on the Report model, so I over-road the test setup filter fields so the test could make it through. Assumes a custom model manager would not affect the FilterField. I only assert when the model manager settings are used.

Also, I updated the quickstart doc to reflect these new settings.