Native 1.2

[mxmCherry]

Not ready yet, submitting pull request just to track progress.

OpenRTB Dynamic Native Ads API Specification Version 1.2 implementation

• 4.1 Native Markup Request Object

• 4.2 Asset Request Object

• 4.3 Title Request Object

• 4.4 Image Request Object

• 4.5 Video Request Object

• 4.6 Data Request Object

• 4.7 Event Trackers Request Object

• 5.1 Native Markup Response Object

• 5.2 Asset Response Object

• 5.3 Title Response Object

• 5.4 Image Response Object

• 5.5 Data Response Object

• 5.6 Video Response Object

• 5.7 Link Response Object

• 5.8 Event Tracker Response Object

• 7.6 Event Types Table

• 7.7 Event Tracking Methods Table

• move enums from native/{request,response}.* -> native.*

• update test files, if they're updated in spec

• update docs (README etc)

• review / double check everything again

[mxmCherry]

Reviewed request/response types once again, no fields seems to be missing.

Haven't reviewed enum values carefully, but these can be fixed later, if anything changed.