B-dynamodb-common

* Improve `pynamodb_list_result` class.

* Model type factory for dynamic table use now supports `Indexes`
(global or local). Previously if you had e.g. GSI in your model,
you would get a "Missing Meta" error.

* Expose `validate_permissions` method.

* Add model type factory to allow dynamic `table_name` and `region` specification.

* Add transformer functions to transform returned results.
* P.S. still not a stable version.

* Add documentation.