.net Core Support!

At long last, we've updated the library for .net core.

This update is mostly compatible with previous versions of the library, but removes support for older versions of .net. We now require at least .net standard 1.1.

The updated package includes some minor improvements/breaking changes related to the inclusion of both a Text and Html body part in some of the convenience sending methods, but depending on how extensive your integration is, should only require some minor modifications to your code.