Airflow-provider-great-expectations

* [BREAKING] Updated GreatExpectations operator to work with class-based Checkpoints (Great Expectations >= 0.13.9)
* [ENHANCEMENT] Restructured project, provider metadata, examples, added "how to run" steps to README, and added tests

* [ENHANCEMENT] Adding BigQuery operator for easier use with BigQuery and Google Cloud Storage

* [ENHANCEMENT] Adding template fields
* [ENHANCEMENT] Adding fail callback function parameter

* [ENHANCEMENT] Updated Great Expectations examples

* Initial release of the provider