Webhooks

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* Convert python-requests bytes to string when using Python 3

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* Replaced `json262` with `standardjson` package.

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* Replaced `utils.encoders` with `json262` package.
* utf-8 encoding everywhere
* Add `from `__future__ import absolute_import` everywhere.

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* Brought in simplified `cached_property` decorator

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* Added more Senderable attributes to make it easier to track what's going on.
* Added the missing webhooks.sender package to setup.py.

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* Added extensible Senderable class to expedite creating new senders.
* Added async_redis sender.
* Added travis-ci.