Common: explore validating the graph for each incoming event

[chuwy]

Currently we validate the types within an event, and we enforce that certain fields in our legacy Tracker Protocol are correctly set, but we don't validate the event semantics as a whole.

To give some examples:

• An iOS screen view lacking a geolocation context is a valid event
• A PagerDuty event consisting of a single entity ("incident_raised") is a valid event
• An event sent by a Snowplow Tracker without a verb is an invalid event
• An ecommerce order without any line items is an invalid event

Relevant writing from Snowplow:

• http://snowplowanalytics.com/blog/2013/08/12/towards-universal-event-analytics-building-an-event-grammar/
• http://snowplowanalytics.com/blog/2014/03/11/building-an-event-grammar-understanding-context/
• http://snowplowanalytics.com/blog/2015/01/18/modeling-events-through-entity-snapshotting/

This looks interesting: http://www.knewton.com/tech/blog/2012/02/graphlint-creating-a-domain-specific-language-for-graph-validation/