Refactor and clarify event status and start/end date verification

[j-d-b]

This PR resolves #189. Specifically, it proposes the following changes:

• Deprecate the event_status property on the WorkZoneRoadEvent and DetourRoadEvent.
• Add deprecation note to EventStatus enumerated type.
• Deprecate the start_date_accuracy and end_date_accuracy properties on the WorkZoneRoadEvent and DetourRoadEvent.
• Add deprecation note to TimeVerification enumerated type.
• Add new boolean properties is_start_date_verified and is_end_date_verified to the WorkZoneRoadEvent and DetourRoadEvent to indicate if the work zone or detour has been confirmed to have started/ended by a person or device in the field or report from a TMC, or similar.
• Require either start_date_accuracy or is_start_date_verified.
• Require eitherend_date_accuracy or is_end_date_verified.
• Update example feeds to use is_start_date_verified and is_end_date_verified and not use event_status, start_date_accuracy, or end_date_accuracy.

See #281 for a similar approach of using booleans that this PR is consistent with.

[sknick-iastate]

@j-d-b I thought we discussed at one point but were we planning on still updating the object and enumeration lists in the spec-content README? Or will we just do this when everything is merged?