Modularize Session Store

[mullermp]

• Prepares moving aws-sessionstore-dynamodb out of required dependencies in the new major version. Railtie will check if it's loaded and if so, apply it.
• Fix tests that were not executing anymore because they rely on minitest. Started a new test directory that will be refactored and combined later.
• Adds a new session store config generator and separates that out from the active record migration.
• Adds a create / delete table rake task for rails apps without active record.
• Improves the documentation and README of the session store feature with rails integration.
• Renamed action dispatch session store configuration to dynamo_db_store to be consistent, and provided a backwards compatible shim with a warning, which will be removed in the new major version.

Closes #139 (feature request for rake create/delete table)