Add Gmail Delegation Commands to PSGSuite v2+

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Email Settings API is currently the only way to perform Gmail Delegation, but the API is deprecated for the newer Gmail API. Gmail API does not yet support delegation tasks though and there is no apparent .NET SDK for the Email Settings API. Expected completion of delegation setting addition to the Gmail API is Q3 of 2018.

Issue to track progress on building a workaround or integrating Gmail API for delegation if released sooner. If you need to perform delegation tasks with PSGSuite, stick to the previous releases until this issue is resolved.

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Ended up refactoring the code for Get-GSToken to remove the dependancy on OWIN for URLSafe Base64 conversion when building the JWT. Tests all running successful for delegation functions in Windows PowerShell and PowerShell Core on both Windows and Ubuntu.

Changes pushed in v2.4.0 to PSGallery!