feature/postgres compatibility, update readme, add source identifiers

[fivetran-catfritz]

Pull Request Are you a current Fivetran customer? Fivetran employee

What change(s) does this PR introduce? Confirm postgres compatibility, update readme to new format, add source identifiers

Did you update the CHANGELOG?

• [x] Yes

Does this PR introduce a breaking change?

• [ ] Yes (please provide breaking change details below.)
• [x] No (please provide an explanation as to how the change is non-breaking below.)

Did you update the dbt_project.yml files with the version upgrade (please leverage standard semantic versioning)? (In both your main project and integration_tests)

• [x] Yes

Is this PR in response to a previously created Bug or Feature Request

• [x] Yes, Issue/Feature #22 and #23
• [ ] No

How did you test the PR changes?

• [x] CircleCi
• [ ] Local (please provide additional testing details below)

Select which warehouse(s) were used to test the PR

• [x] BigQuery
• [x] Redshift
• [x] Snowflake
• [x] Postgres
• [x] Databricks
• [ ] Other (provide details below)

Provide an emoji that best describes your current mood

:zap: **Feedback** We are so excited you decided to contribute to the Fivetran community dbt package! We continue to work to improve the packages and would greatly appreciate your [feedback](https://www.surveymonkey.com/r/DQ7K7WW) on our existing dbt packages or what you'd like to see next.

[fivetran-catfritz]

@fivetran-joemarkiewicz Changes have been incorporated. Thanks!