Merge branch 'feature/webhooks-3'
Patched: com.pagerduty/incident/jsonschema/1-0-0 (closes #78)
PagerDuty Incident Schema fixed for wrong key types
Updated mandrill message_opened schema as per https://github.com/snowplow/iglu-central/issues/79
Updated message_open schema as per https://github.com/snowplow/iglu-central/issues/80
Updated mandrill message_open schema as per https://github.com/snowplow/iglu-central/issues/81
Moved smtp_events, template and resends fields into msg object (nested properties)
Patched com.mandrill.message_opened as per https://github.com/snowplow/iglu-central/issues/82
Patched message_sent schema
Patched com.mandrill/message_clicked schema
Patched com.mandrill/message_rejected schema
Updated schema of nested smtp_events objects in all Mnadrill objects, based on fields found in one fo the event types. (Assume Mandrill has a consistent model of that event type acros all its different events
Patched mandrill message bounced schema
Patched mandrill message_soft_bounced schema
Added Client field to PagerDuty schema
Added schema for a com.pingdom/incident_notify_user event (closes #86)
Patched mailchimp subscribe schema
Revert "Patched mailchimp subscribe schema"

This reverts commit b63cbefb15136f1d63ffdb90e7861bfe0da98d71.
Patched MailChimp subscribe schema

- Small patch to allow more information to flow through the “merges”
field.
Merge branch 'feature/webhooks-3' of https://github.com/snowplow/iglu-central into feature/webhooks-3