Loafer

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* Enable provider parameters (boto client options)

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* Bugfix: fix SNS prefix value in use for topic name wildcard (by lamenezes)

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* Added initial contracsts for class-based handlers
* Added generic handlers: SQSHandler/SNSHander
* Improve internal error handling
* Improve docs

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* Fix sentry error handler integration

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* Add tox and execute tests for py36
* Update aiohttp/aiobotocore versions
* Minor fixes and enhancements

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* Major code rewrite
* Remove CLI
* Add better support for error handlers, including sentry/raven
* Refactor exceptions
* Add message metadata information
* Update message lifecycle with handler/error handler return value
* Enable execution of one service iteration (by default, it still runs "forever")