API Keys support

[thinkingserious]

Make sure all examples and documentation is updated:

https://azure.microsoft.com/en-us/documentation/articles/sendgrid-dotnet-how-to-send-email/
https://github.com/sendgrid/sendgrid-csharp
https://sendgrid.com/docs/Integrate/Code_Examples/csharp.html

[antitheos]

I am trying to use version 6.2.0, but I am not able to find the constructor for new Web(api_key)
If there an additional piece I am missing?

[thinkingserious]

Hello @antitheos,

We have made the update, but need to merge the pull request: #127

[thinkingserious]

@antitheos

Update complete: #164

Thanks for your patience :)