Tgbotplug

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* Make use of `logging`

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* Small bugs fixed

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* plugintest also saves sent files, for unit test asserts
* support for webhook responses for `sendMessage` API method only: plugins methods can return `self.bot.return_message` instead of calling `self.bot.send_message` - if the call came from a webhook it will then be returned in the HTTP response body, otherwise normal API call will be issued

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* plugintest module completely re-done

Impact - only in your plugin test cases:
* PluginTestCase().clear_replies() renamed to clear_queues()
* PluginTestCase().assertReplied() parameters changed order

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* Bug fix to allow existing User and GroupChat instances to be updated (instead of ignored) - issue 22
* Bug fixes - issue 23 and 24
* `print_commands` now lists commands in the same order they are declared in `list_commands` - issue 18

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* Quick fix for plugin re-attachment to the same bot instance