Boomerang

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- **BREAKING CHANGE:** Handler functions are no longer overridden in the
``Messenger`` class; instead, the ``app.handle`` decorator is used. See the
documentation for more details.
- Handler functions can now simply return responses in a variety of formats,
which are interpreted and sent as a reply.

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- Implement the User Profile API.

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- Implement the Thread Settings API.

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- Add automatic attachment hosting using the internal server
- Add proper handling of Messenger API errors

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- Update dependency versions to fix VersionConflict in Travis CI.

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- Implement the Send API. All non-beta templates and messages are supported
(except for the airline templates).