[x] Adds a "simplify and resend" feature for soft-fail responses.
[x] Rewrite of logging, to use osTicket inbuilt logging system. Admin -> Settings -> System -> "Default Log Level". Set it to "DEBUG" to see the plugin working things out in your normal "Admin -> Dashboard -> System Logs"
[x] Test!
[x] Needs a bit of a cleanup, getting scraggly.
[ ] Message Queue.. there's a fair few, but the external ones depend on quite a few things and are likely not available, we could simply add a db_table and use that.. might be for the best.
Relates to https://github.com/thammanna/osticket-slack/pull/11#issuecomment-356086426
Hopefully correctly handles the possible/likely HTTP return codes from https://api.slack.com/changelog/2016-05-17-changes-to-errors-for-incoming-webhooks