Add Slack plugin, add core functionality for oauth authorization

[gigxz]

Closes #52

Instead of adding each Slack action and event manually, there is a single action called slack.method for performing any Slack method https://api.slack.com/methods

Core changes to support Slack are here: 650ed98

Deferred follow-ups:

• Come up with a better way for the "global" handlers to be registered. Right now core just looks for a views.py file in the plugin, with specific function names (get_authorize_url, auth_callback, and process_event). And document it.
• The Slack "config" options should not be visible client side, since they get filled in via the oauth flow. Add flags for different kinds of config options.
• Create a better way for plugins to define "server-side global settings" instead of just env files
• Create a GovernanceProcess for Slack voting
• Tests for Slack plugin