Separate Nuget package for Unity helpers

[timcassell]

RestClient for Unity is interested in adopting ProtoPromise proyecto26/RestClient#216. They have a Nuget package with UnityEngine dependencies. The ProtoPromise Nuget package does not have any UnityEngine dependencies so that it can work on any .Net platform.

We do have some UnityEngine depencies in the UnityPackage for helpers like converting coroutines to Promises and executing the PromiseSynchronizationContext for foreground scheduling. Obviously, I would recommend just using the UnityPackage for source code, but we should create a separate Nuget package that contains those Unity helpers with the UnityEngine.dll dependency so that RestClient and other Unity-based libraries who wish to use ProtoPromise and host their package on Nuget can add a dependency to that helper package.