[WIP] Add Support for deterministic UUIDs

[fabiopelosin]

This is the work in progress patch for #175.

The difficult part of this patch are:

• Xcodeproj relies on the UUIDs (objects_by_uuid) for its internal reference counting.
• Proxies to avoid retain cycles and to allow to reference objects in another projects consider the UUIDs as strings (instead of using the Xcodeproj reference system) and objects, with this patch, might not have an UUID until saved.

There are alternative approaches like specifying an temporary UUID during the creation of the object and then update it during save according to the final attributes of the object. However, I feel that this approach would be slower and I would prefer to first explore any alternative with the approach proposed in this patch.

550 specifications (886 requirements), 3 failures, 0 errors

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Closes CocoaPods/CocoaPods#1118

[kylef]

Hey, I'm going to close this as nothing is moving in a while. Please re-open when this is ready for merge.