Move rot function down into turbot/commands.py

Move rot function down into turbot/commands.py

Completing the bulk of the code movement out of turbot_lambda.py

Now, the code in turbot_lambda is really just about handling the
request and figuring out which type it is, (whether a Slack action,
command, event, etc.), and calling out to the appropriate code in
turbot.actions, turbot.commands, turbot.events, etc.