Preliminary architecture for supporting webhooks
New webhook.go - defines the structure and interface for a webhook. Almost a carbon copy of webhook spec from here but with some getters and setters.
New events.go - a combinations of reviewboard's reviewboard.reviews.signals and django's django.dispatch.signal, it defines a global of all the events that rb-gateway supports.
routes.go - added 2 new api routes. 1 for registering a new webhook and 1 for accessing/editing webhooks.
config.json - supports new config keys for defining webhooks
util.go - added new functions for creating webhooks and editing them

None yet but what is planned:
webhook_test.go

Test notify in webhook

Test get json in webhook

Test getters/setters
util_test.go

Test webhook/event/repo portion of TestLoadConfig in util_test.go

Test register webhook in util

Test delete webhook in util

Test save webhook in util

Test refresh config in util
events_test.go

Test get receivers

Test send

Test subscribe/unsubscribe