This removes the reference to the default region being us-east-1 if not specified in the Model's Meta class. This is no longer true as of version 6.0.0.

…db#1271)

This adds the ability to directly set the boto retry configuration
dictionary, or to leave it unset and allow botocore to automatically
discover the configuration from the environment or `~/.aws/config`
files.

The default is to use the previous PynamoDB behavior for configuring
retries so as to not make this a breaking change.

This adds a `wait` argument that will make the `delete_table` call block
until the DynamoDB table is fully deleted if set to `True`. It defaults
to `False` to not break the existing behavior.