Add support for IDFA/GAID in spec

[Niek]

There are a few ways to track mobile devices: IDFA (iOS), GAID (Android), WAID (Windows) and BBID (Blackberry). None of these are implemented in the OpenRTB spec, there is only device.ifa (very vague description) and device.dpidmd5 / device.dpidsha1 (applies only to the pre-2013 AndroidID/UDID).

Is it possible to make one of the following changes?

• Describedevice.ifa in such a way that it's clear this is meant to contain the IDFA/GAID/WAID/BBID depending on the device OS, or:
• Create a new device.adid (or similar name) that contains the IDFA/GAID/WAID/BBID depending on the device OS, or:
• Create separate device.idfa, device.gaid, device.waid entries device.bbid containing the ID