Add interfaces to support SOLID

[roryprimrose]

I would like to use this library, however the lack of interfaces means that I would have to tightly couple my classes to the SendGrid classes. Please add interfaces to the classes to allow for loose coupling.

[dracan]

Can't you just abstract the calls out into your own wrapper interfaces/classes? If they add interfaces, then you're still coupled to their interfaces.

[roryprimrose]

Yes, that is true. There is always going to be some code of mine that links into theirs. I would rather be coupled to an interface that I can inject into a constructor than an implementation (albeit an abstract one).

[thinkingserious]

@roryprimrose Thanks for the suggestion!

We will keep this on our backlog for now.

[thinkingserious]

This is note for anyone who wished to contribute on this:

Please add pull requests to this branch: https://github.com/sendgrid/sendgrid-csharp/tree/v3beta and we would need a signed CLA: https://github.com/sendgrid/sendgrid-csharp/blob/v3beta/CONTRIBUTING.md#cla

[M3LiNdRu]

The following PR, it has started fixing the issue

See: #350