Simple-aws-rds

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**Minor Improvements**

- Now ``RDSDBInstance.status`` attribute is optional.

**Bugfixes**

- fix a bug that ``RDSDBSnapshot.wait_for_status`` uses the wrong status enum class.

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**Bugfixes**

- Fix a bug that waiter method does not respect ``gap`` and ``instant`` parameter.

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**Features and Improvements**

- add ``RDSDBSnapshot``

**Minor Improvements**

- the waiter method now can do the check instantly.
- add document website.

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**Features and Improvements**

- add ``RDSDBInstance.delete_db_instance``

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**Features and Improvements**

- add ``RDSDBInstanceStatusGroupEnum``
- add ``RDSDBInstance.wait_for_available``
- add ``RDSDBInstance.wait_for_stopped``

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**Features and Improvements**

- add ``RDSDBInstnace.wait_for_status`` method

**Minor Improvements**

- add a lot more attributes to ``RDSDBInstnace``
- add a lot more status enum to ``RDSDBInstanceStatusEnum``