fix Slack notification and support apps/webhooks

[armysick]

poshc2/server/Implant.py was lacking import of json , making conn.request("POST", connurl,json.dumps(message), headers) throw an Exception.

Usage of "https://[YourSlackName].slack.com/apps/A0F7YS25R" is also no longer recommended by Slack and I failed to make it work, so I've developed support for a Slack App/Bot using Slack WebHooks (hooks.slack.com) instead.