delete slack messages and files. An improved version is at:
conversation api throughout the code to avoid the deprecated channels, ims, groups, and mpim apisintegrates #75 which iterates over messages replies properly. Moreover, it handles tombstone messages which are generated when one deletes the thread start message.
first version that is published via pypi thanks to the migration of the project to my account.
closes #47 by using the conversation api since converted public channels doesn't show up as groups.