Payload and device token validations

[nathany]

These validations are brought over from a closed source library. All of these validations are opt-in. They aren't performed automatically.

• APS notifications can contain just custom data without a badge or body, but Buford has no way of knowing if custom data will be added later.
• Passbook notifications are completely empty, but it doesn't really make sense to use APS or even serialize JSON when you can just use PushBytes with {}.
• Newsstand notifications are empty except for ContentAvailable. APS won't validate a payload like this right now. Newsstand isn't really a thing anymore, but maybe there are other cases where only ContentAvailable should be set? This could be another case where serialization can be skipped.
• ContentAvailable-only notifications must be used with the LowPriority header, but these are set in different packages so no attempt is made to validate this.
• Device tokens may get larger in the future. In 2016, [device tokens] may be growing up to 100 bytes. #13

These validations should probably be verified against the Apple documentation to make sure they are still the correct thing to do.

I haven't done anything to validate the push Headers yet, such as the UUID format for the ID or that the expiration is in the future.

[curtisallen]

Awesome LGTM
thanks!