Add Librato integration.

Fixes: #68.

zerver/lib/integrations.py

@@ -87,6 +87,7 @@ WEBHOOK_INTEGRATIONS = [
     WebhookIntegration('helloworld', display_name='Hello World'),
     WebhookIntegration('ifttt', function='zerver.views.webhooks.ifttt.api_iftt_app_webhook', display_name='IFTTT'),
     WebhookIntegration('jira', secondary_line_text='(hosted or v5.2+)', display_name='JIRA'),
+    WebhookIntegration('librato'),
     WebhookIntegration('newrelic', display_name='New Relic'),
     WebhookIntegration('pagerduty'),
     WebhookIntegration('pingdom'),